fix(client): degrade against a node still speaking MNP 1.0

The SPA is served by the hub, so deploying the hub puts this client in front of
every node — including the ones not updated yet. That window is the normal
state for as long as an operator takes, and for a node someone else runs it may
be indefinite. Three controls were broken across it, and the failure mode is
quiet: an unknown message type is logged by the node and never answered, so the
click produces a thirty-second wait ending in a timeout with nothing on screen
to say the node simply cannot do this.

Files' Upload button read `root.writable`, which a 1.0 node does not send — it
says `upload`, the same answer under the older name. The button disappeared on
every un-upgraded node. It reads both now, and still respects an explicit
`writable: false` rather than falling through to the legacy flag.

The per-app folder pickers spoke `app_directories`. Videos, Music and Photos
each had their own message before that and those still work, so the page
chooses by version: an operator on an older node keeps the ability they had.
`video_root` and `audio_root` hold one folder, so several are refused with a
reason rather than stored as the first and silently truncated.

Root management — writable, removable, eject, plug — has no older equivalent to
route to, so the table goes read-only with a line saying why and pointing at
the `meshbay-node root` commands. Chat's two settings are new with nothing
before them and are hidden the same way.

None of this was inferred from a payload's shape: `_checkNodeVersion` already
parsed the node's version and threw it away, and it is kept now. Coupling a
capability to whether some field happens to be present is how a flag flips
because an unrelated payload changed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pvMdvLBG92jyhvD5pD6us

14 files changed, 108 insertions, 4 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/files-app.js b/packages/meshbay-hub/src/meshbay_hub/static/files-app.js
index 6825c8d..a6a3ca4 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/files-app.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/files-app.js
@@ -221,7 +221,14 @@ function FilesPanel({
     : subdirs.filter(d => rootState.get(d) && rootState.get(d).available === false);
   const currentRootName = currentPath ? currentPath.split('/')[0] : '';
   const currentRoot = currentRootName ? rootState.get(currentRootName) : null;
-  const currentRootWritable = currentRoot ? currentRoot.writable : false;
+  // `upload` is the same answer under the name a node speaking MNP 1.0 uses;
+  // reading only `writable` there means the Upload button disappears on every
+  // node that has not been upgraded yet, which is most of them on the day the
+  // page ships.
+  const currentRootWritable = currentRoot
+    ? (currentRoot.writable !== undefined ? currentRoot.writable
+                                          : Boolean(currentRoot.upload))
+    : false;
   // A member cannot create a folder at the top of a group: that level is the
   // set of roots, which is the operator's configuration and not a directory on
   // anyone's disk. The node refuses it, so offering it would only produce an
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/group-page.js b/packages/meshbay-hub/src/meshbay_hub/static/group-page.js
index e762fc8..513792b 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/group-page.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/group-page.js
@@ -140,6 +140,10 @@ function GroupPage({ groupId, group, token, username, userId, userPrefs,
   // destination rather than a set of folders it reads.
   const [chatDirectory, setChatDirectory] = useState('');
   const [chatLinkPreview, setChatLinkPreview] = useState(true);
+  // Whether this node speaks the operations MNP 1.1 added. False for one that
+  // predates them, and the Settings page then offers what that node can
+  // actually do rather than controls whose messages it drops unanswered.
+  const [nodeSupportsAppOps, setNodeSupportsAppOps] = useState(false);
   // MusicBrainz on/off (per-group) — docs/musicbay.md §3.2.
   const [musicbrainzConfig, setMusicbrainzConfig] = useState(null);
   const onPlayQueue = useCallback((tracks, startIndex) => {
@@ -342,6 +346,7 @@ function GroupPage({ groupId, group, token, username, userId, userPrefs,
             || (ack.audio_root ? [ack.audio_root] : []),
           photo: ack.photo_directories || ack.photo_roots || [],
         });
+        setNodeSupportsAppOps(transport.supportsAppOps);
         setChatDirectory(ack.chat_directory || '');
         setChatLinkPreview(ack.chat_link_preview !== false);
         setMusicbrainzConfig({
@@ -790,6 +795,7 @@ function GroupPage({ groupId, group, token, username, userId, userPrefs,
             onScanSettings=${(s) => setScanSettings(s)}
             entries=${entries} nodeDirs=${nodeDirs}
             appSettings=${appSettings}
+            nodeSupportsAppOps=${nodeSupportsAppOps}
             ${/* The saving pane already knows what it asked for; this is so
                   the page's own copy moves at the same time, rather than
                   waiting for the ack it will not be handed (transport.js
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/group-settings.js b/packages/meshbay-hub/src/meshbay_hub/static/group-settings.js
index 5d0ab07..f8b4aa5 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/group-settings.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/group-settings.js
@@ -361,7 +361,8 @@ function GroupSettingsPanel({ groupId, group, token, transportRef, gekRef,
                              enabledApps, onEnabledApps,
                              scanSettings, onScanSettings,
                              entries, nodeDirs,
-                             appSettings, onAppDirectories, onRefreshIndex,
+                             appSettings, nodeSupportsAppOps,
+                             onAppDirectories, onRefreshIndex,
                              onPaired, onLeft }) {
   const [members, setMembers] = useState([]);
   const [adminId, setAdminId] = useState('');
@@ -647,7 +648,14 @@ function GroupSettingsPanel({ groupId, group, token, transportRef, gekRef,
     if (!transport || !transport.connected) {
       throw new Error(t('node.root_no_route'));
     }
-    await transport.setAppDirectories(appKey, paths, adminSignFn);
+    // A node too old for the generic op still answers the three per-app
+    // messages that came before it, so an operator on one keeps the ability
+    // they had rather than being handed a control that times out.
+    if (transport.supportsAppOps) {
+      await transport.setAppDirectories(appKey, paths, adminSignFn);
+    } else {
+      await transport.setAppDirectoriesLegacy(appKey, paths, adminSignFn);
+    }
     if (onAppDirectories) onAppDirectories(appKey, paths);
   }, [transportRef, adminSignFn, onAppDirectories]);
 
@@ -845,11 +853,19 @@ function GroupSettingsPanel({ groupId, group, token, transportRef, gekRef,
           <p class="settings-hint">${t('settings_node.shared_directories_hint')}</p>
           ${!connected && nodeDetected && html`
             <p class="settings-hint">${t('settings_node.roots_offline_hint')}</p>`}
+          ${/* Read-only against a node that predates the root operations:
+                writable, removable, eject and plug have no older equivalent
+                to fall back to, and an unknown message type is dropped
+                unanswered — a thirty-second wait ending in a timeout, with
+                nothing on screen to say the node simply cannot do it. */''}
+          ${connected && !nodeSupportsAppOps && !nodeDetected && html`
+            <p class="settings-hint">${t('settings_node.roots_node_too_old')}</p>`}
           <${SharedDirectoriesTable}
             roots=${effectiveRoots}
             groupId=${groupId}
             transport=${transportRef.current}
             signFn=${adminSignFn}
+            readOnly=${connected && !nodeSupportsAppOps && !nodeDetected}
             nodeDetected=${nodeDetected}
             onRootsChange=${loadNodeInfo}
             onRefreshIndex=${onRefreshIndex} />
@@ -876,7 +892,9 @@ function GroupSettingsPanel({ groupId, group, token, transportRef, gekRef,
             disabled=${appsBusy}
             onChange=${() => toggleApp(app.key)} />
         `}>
-          ${activeApps.includes(app.key)
+          ${!nodeSupportsAppOps && app.key === 'chat'
+            ? html`<p class="settings-hint">${t('settings_node.app_node_too_old')}</p>`
+            : activeApps.includes(app.key)
             ? html`<${app.Settings}
                 roots=${effectiveRoots} dirs=${folderOptions}
                 settings=${appSettings}
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/de.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/de.js
index d5d1429..edaa70a 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/de.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/de.js
@@ -846,6 +846,8 @@ export default {
   'settings_app.tmdb_token_prompt': 'Registrieren Sie sich bei TMDB, um einen eigenen API-Schlüssel zu erzeugen.',
   'settings_app.tmdb_token_link': 'Schlüssel holen',
   'settings_node.roots_offline_hint': 'Nicht mit dem Node verbunden — Änderungen laufen über den lokalen Node und greifen beim nächsten Neuladen.',
+  'settings_node.roots_node_too_old': 'Dieser Node ist älter als diese Seite: Er kann seine Verzeichnisse anzeigen, aber hier nicht ändern. Aktualisieren Sie ihn oder nutzen Sie die meshbay-node root-Befehle.',
+  'settings_node.app_node_too_old': 'Dieser Node ist älter als diese Seite und hat für diese App noch keine Einstellung. Aktualisieren Sie ihn, um sie hier zu konfigurieren.',
   'settings_node.directories_title': 'App-Verzeichnisse',
   'settings_node.directories_hint': 'Freigegebene Ordner und welchen davon die Videos-, Musik- und Fotos-Apps als eigene(n) Einstiegspunkt(e) nutzen.',
 
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js
index 8c5cbe5..d1aca93 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js
@@ -634,6 +634,8 @@ export default {
   'settings_app.tmdb_token_prompt': 'Sign up on TMDB to generate your own API key.',
   'settings_app.tmdb_token_link': 'Get a key',
   'settings_node.roots_offline_hint': 'Not connected to the node — changes go through the local node instead, and take effect on its next reload.',
+  'settings_node.roots_node_too_old': 'This node is older than this page: it can show its directories but not change them here. Update it, or use the meshbay-node root commands.',
+  'settings_node.app_node_too_old': 'This node is older than this page and has no setting for this app yet. Update it to configure this here.',
   'settings_node.directories_title': 'App directories',
   'settings_node.directories_hint': 'Which shared folders the Videos, Music and Photos apps use as their entry point(s).',
 
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/es.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/es.js
index 4a7384f..880de2e 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/es.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/es.js
@@ -842,6 +842,8 @@ export default {
   'settings_app.tmdb_token_prompt': 'Regístrate en TMDB para generar tu propia clave de API.',
   'settings_app.tmdb_token_link': 'Obtener una clave',
   'settings_node.roots_offline_hint': 'Sin conexión con el nodo: los cambios pasan por el nodo local y se aplican en su próxima recarga.',
+  'settings_node.roots_node_too_old': 'Este nodo es más antiguo que esta página: puede mostrar sus directorios pero no cambiarlos aquí. Actualízalo o usa los comandos meshbay-node root.',
+  'settings_node.app_node_too_old': 'Este nodo es más antiguo que esta página y aún no tiene ajustes para esta aplicación. Actualízalo para configurarla aquí.',
   'settings_node.directories_title': 'Directorios de apps',
   'settings_node.directories_hint': 'Carpetas compartidas, y cuál de ellas usan las apps de Vídeos, Música y Fotos como su(s) propio(s) punto(s) de entrada.',
 
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js
index bee4ae9..eb977f3 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js
@@ -860,6 +860,8 @@ export default {
   'settings_app.tmdb_token_prompt': 'Créez un compte TMDB pour générer votre propre clé d\'API.',
   'settings_app.tmdb_token_link': 'Obtenir une clé',
   'settings_node.roots_offline_hint': 'Non connecté au nœud — les changements passent par le nœud local et prennent effet à son prochain rechargement.',
+  'settings_node.roots_node_too_old': 'Ce nœud est plus ancien que cette page : il peut afficher ses répertoires mais pas les modifier ici. Mettez-le à jour, ou utilisez les commandes meshbay-node root.',
+  'settings_node.app_node_too_old': 'Ce nœud est plus ancien que cette page et n\'a pas encore de réglage pour cette application. Mettez-le à jour pour la configurer ici.',
   'settings_node.directories_title': 'Répertoires des applications',
   'settings_node.directories_hint': 'Quel(s) dossier(s) partagés les applications Vidéos, Musique et Photos utilisent comme leur(s) propre(s) point(s) d\'entrée.',
 
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js
index e39d91d..2f58015 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js
@@ -856,6 +856,8 @@ export default {
   'settings_app.tmdb_token_prompt': 'Registrati su TMDB per generare la tua chiave API.',
   'settings_app.tmdb_token_link': 'Ottieni una chiave',
   'settings_node.roots_offline_hint': 'Non connesso al nodo: le modifiche passano dal nodo locale e hanno effetto al successivo ricaricamento.',
+  'settings_node.roots_node_too_old': 'Questo nodo è più vecchio di questa pagina: può mostrare le sue directory ma non modificarle qui. Aggiornalo, oppure usa i comandi meshbay-node root.',
+  'settings_node.app_node_too_old': 'Questo nodo è più vecchio di questa pagina e non ha ancora impostazioni per questa applicazione. Aggiornalo per configurarla qui.',
   'settings_node.directories_title': 'Directory delle app',
   'settings_node.directories_hint': 'Cartelle condivise, e quale di esse le app Video, Musica e Foto usano come proprio/i punto/i di ingresso.',
 
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/ja.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/ja.js
index 75a3aa2..f22ce21 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/ja.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/ja.js
@@ -840,6 +840,8 @@ export default {
   'settings_app.tmdb_token_prompt': 'TMDB に登録して、自分の API キーを発行してください。',
   'settings_app.tmdb_token_link': 'キーを取得',
   'settings_node.roots_offline_hint': 'ノードに接続していません — 変更はローカルノード経由で行われ、次回の再読み込みで反映されます。',
+  'settings_node.roots_node_too_old': 'このノードはこのページより古く、ディレクトリの表示はできますがここでの変更はできません。更新するか、meshbay-node root コマンドを使ってください。',
+  'settings_node.app_node_too_old': 'このノードはこのページより古く、このアプリの設定をまだ持っていません。ここで設定するには更新してください。',
   'settings_node.directories_title': 'アプリのディレクトリ',
   'settings_node.directories_hint': '共有フォルダと、動画・音楽・写真の各アプリがそれぞれの起点として使用するフォルダです。',
 
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/nl.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/nl.js
index 7070288..306cf44 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/nl.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/nl.js
@@ -858,6 +858,8 @@ export default {
   'settings_app.tmdb_token_prompt': 'Meld u aan bij TMDB om uw eigen API-sleutel te maken.',
   'settings_app.tmdb_token_link': 'Sleutel ophalen',
   'settings_node.roots_offline_hint': 'Niet verbonden met de node — wijzigingen gaan via de lokale node en worden bij de volgende herlaadbeurt actief.',
+  'settings_node.roots_node_too_old': 'Deze node is ouder dan deze pagina: hij kan zijn mappen tonen maar hier niet wijzigen. Werk hem bij, of gebruik de meshbay-node root-opdrachten.',
+  'settings_node.app_node_too_old': 'Deze node is ouder dan deze pagina en heeft nog geen instelling voor deze app. Werk hem bij om die hier in te stellen.',
   'settings_node.directories_title': 'App-mappen',
   'settings_node.directories_hint': 'Gedeelde mappen, en welke daarvan de Video\'s-, Muziek- en Foto\'s-apps als eigen startpunt(en) gebruiken.',
 
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/pl.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/pl.js
index c252e82..f2b3faf 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/pl.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/pl.js
@@ -884,6 +884,8 @@ export default {
   'settings_app.tmdb_token_prompt': 'Zarejestruj się w TMDB, aby wygenerować własny klucz API.',
   'settings_app.tmdb_token_link': 'Pobierz klucz',
   'settings_node.roots_offline_hint': 'Brak połączenia z węzłem — zmiany przechodzą przez węzeł lokalny i zaczną działać po jego następnym przeładowaniu.',
+  'settings_node.roots_node_too_old': 'Ten węzeł jest starszy niż ta strona: może pokazać swoje katalogi, ale nie zmieni ich tutaj. Zaktualizuj go albo użyj poleceń meshbay-node root.',
+  'settings_node.app_node_too_old': 'Ten węzeł jest starszy niż ta strona i nie ma jeszcze ustawień tej aplikacji. Zaktualizuj go, aby skonfigurować ją tutaj.',
   'settings_node.directories_title': 'Katalogi aplikacji',
   'settings_node.directories_hint': 'Katalogi udostępnione oraz to, który z nich aplikacje Wideo, Muzyka i Zdjęcia traktują jako własny punkt (punkty) wejścia.',
 
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js
index 7fb6c8c..e608fbd 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/pt-BR.js
@@ -843,6 +843,8 @@ export default {
   'settings_app.tmdb_token_prompt': 'Cadastre-se no TMDB para gerar sua própria chave de API.',
   'settings_app.tmdb_token_link': 'Obter uma chave',
   'settings_node.roots_offline_hint': 'Sem conexão com o nó — as alterações passam pelo nó local e entram em vigor no próximo recarregamento.',
+  'settings_node.roots_node_too_old': 'Este nó é mais antigo que esta página: ele pode mostrar seus diretórios, mas não alterá-los aqui. Atualize-o ou use os comandos meshbay-node root.',
+  'settings_node.app_node_too_old': 'Este nó é mais antigo que esta página e ainda não tem configuração para este aplicativo. Atualize-o para configurá-lo aqui.',
   'settings_node.directories_title': 'Diretórios de apps',
   'settings_node.directories_hint': 'Pastas compartilhadas, e qual delas os apps Vídeos, Música e Fotos tratam como seu(s) próprio(s) ponto(s) de entrada.',
 
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/locales/zh-CN.js b/packages/meshbay-hub/src/meshbay_hub/static/locales/zh-CN.js
index e0886a2..68d794e 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/zh-CN.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/zh-CN.js
@@ -827,6 +827,8 @@ export default {
   'settings_app.tmdb_token_prompt': '在 TMDB 注册以生成你自己的 API 密钥。',
   'settings_app.tmdb_token_link': '获取密钥',
   'settings_node.roots_offline_hint': '未连接到节点 — 变更将通过本地节点进行，并在其下次重新加载时生效。',
+  'settings_node.roots_node_too_old': '该节点比本页面旧：它能显示自己的目录，但无法在此更改。请更新节点，或使用 meshbay-node root 命令。',
+  'settings_node.app_node_too_old': '该节点比本页面旧，尚不支持此应用的设置。请更新节点后在此配置。',
   'settings_node.directories_title': '应用目录',
   'settings_node.directories_hint': '共享文件夹，以及“视频”“音乐”和“照片”应用各自使用哪个（些）作为入口。',
 
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/transport.js b/packages/meshbay-hub/src/meshbay_hub/static/transport.js
index 785b926..32f8539 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/transport.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/transport.js
@@ -335,6 +335,25 @@ class MeshBayTransport {
   set onIndexDelta(fn) { this._onIndexDelta = fn; }
   set onUploadPolicy(fn) { this._onUploadPolicy = fn; }
   set onRootsChanged(fn) { this._onRootsChanged = fn; }
+
+  /** The MNP version the connected node declared, or '' before a handshake. */
+  get nodeVersion() { return this._nodeVersion || ''; }
+
+  /**
+   * Whether the node speaks the per-root and per-app operations MNP 1.1 added:
+   * `root_update`/`root_eject`/`root_plug`, `app_directories`,
+   * `chat_directory`, `chat_link_preview`.
+   *
+   * An older node has no equivalent for the root ones at all, and answers the
+   * app ones through their three predecessors (`video_root`, `audio_root`,
+   * `photo_roots`). The caller chooses which; what it must not do is send a
+   * 1.1 message and wait, because an unknown type is logged and dropped.
+   */
+  get supportsAppOps() {
+    const m = /^(\d+)\.(\d+)$/.exec(this._nodeVersion || '');
+    if (!m) return false;
+    return (Number(m[1]) > 1) || (Number(m[1]) === 1 && Number(m[2]) >= 1);
+  }
   set onAppsEnabled(fn) { this._onAppsEnabled = fn; }
   set onAppDirectories(fn) { this._onAppDirectories = fn; }
   set onChatDirectory(fn) { this._onChatDirectory = fn; }
@@ -600,6 +619,12 @@ class MeshBayTransport {
       // block, because everything below — the join, the proof, the sealed ack
       // — assumes both sides mean the same thing by each message.
       _checkNodeVersion(reply);
+      // Kept, not just checked. Several controls exist only on a node new
+      // enough to have them, and the alternative to asking is offering a
+      // button whose message an older node logs as unknown and never answers
+      // — a 30-second wait ending in a timeout, with nothing on screen to say
+      // the node simply cannot do this.
+      this._nodeVersion = String(reply.v || '');
       if (!window.MeshBayCrypto) {
         throw new Error('Node requires GEK proof but no crypto available');
       }
@@ -1195,6 +1220,34 @@ class MeshBayTransport {
    * Cleaned and sorted the same way the node does, so both sides build the
    * same bytes to sign.
    */
+  /**
+   * The same instruction a node too old for `app_directories` understands.
+   *
+   * Videos, Music and Photos each had their own message before this, and they
+   * still work — so an operator on an un-upgraded node keeps the ability they
+   * had, rather than being handed a control that silently times out. Chat has
+   * no predecessor, which is why its settings are hidden rather than routed.
+   */
+  async setAppDirectoriesLegacy(appKey, directories, signFn) {
+    const clean = [...new Set(
+      (directories || []).map((d) => (d || '').replace(/^\/+|\/+$/g, '')).filter(Boolean),
+    )].sort();
+    if (appKey === 'photo') return this.setPhotoRoots(clean, signFn);
+    // One folder was all these two could carry. Sending several would store
+    // the first and silently drop the rest, so it is refused instead.
+    if (clean.length > 1) {
+      throw new Error(
+        'This node is older than this page and can hold one folder per app. '
+        + 'Update it, or choose a single folder.');
+    }
+    const one = clean[0] || '';
+    if (appKey === 'video') return this.setVideoRoot(one, signFn);
+    if (appKey === 'music') return this.setAudioRoot(one, signFn);
+    throw new Error(
+      'This node is older than this page and cannot store this app\'s '
+      + 'folders. Its operator has to update it.');
+  }
+
   async setAppDirectories(appKey, directories, signFn) {
     const clean = [...new Set(
       (directories || []).map((d) => (d || '').replace(/^\/+|\/+$/g, '')).filter(Boolean),