fix(client): downloads stream to disk, and two rough edges on first run

**Downloads were going through RAM.** `_openDownloadTarget` tries a granted
folder, then a service worker, then its floor: collect the whole file in the
page and hand the browser a blob. Both of the first two are absent in the
desktop application — `showDirectoryPicker` does not exist, and Chromium refuses
a service worker on a custom scheme — so every download under 512 MB took the
floor. A gigabyte of film meant a gigabyte of RAM, and the only visible symptom
was a Save As dialog at the *end* rather than the start, which is what the
operator noticed and asked about.

The main process now streams to disk: it honours "save automatically" with a
folder chosen once and no dialog, never overwrites (a colliding name gets a
suffix), awaits each write so the renderer cannot outrun the disk and queue the
file in memory anyway, and unlinks a cancelled download rather than leaving a
truncated file that looks complete to whoever opens it next. Settings now offers
the native folder picker instead of saying downloads are unsupported.

Measured in the running application: the file on disk grows 256 KB → 512 KB →
768 KB → 1 MB as the chunks arrive, and an aborted download leaves nothing
behind.

**A permanent scrollbar on sign-in.** `.layout` and `.page-center` each reserved
`100vh - 52px`, and `.page-center` sits inside `main`'s 24px vertical padding —
so the page overflowed by exactly 48px at every window size. Found by measuring
in the app rather than reading the stylesheet: `scrollHeight` 819 against a 771
viewport, then the bottom edge of every element. The centring page brings its
own padding, so main's is dropped for it and the duplicated arithmetic goes
rather than growing a third term. Now `scrollHeight == innerHeight`, no
overflowing elements.

**The first-run screen was unstyled.** It used a class name I invented
(`auth-page`) that appears nowhere in the stylesheet, so it had no card and the
button sat against the input. It now uses the same `page-center` + `login-card`
markup as sign-in, which is where the 12px gap comes from. The sign-in link in
the nav is hidden until a hub is chosen — it led to a page that could not work.

809 tests pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

13 files changed, 96 insertions, 23 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/app.js b/packages/meshbay-hub/src/meshbay_hub/static/app.js
index 4091ae7..f49d412 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/app.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/app.js
@@ -569,7 +569,8 @@ function TransferWidget() {
 
 // ── Nav ──────────────────────────────────────────────────────────────────────
 
-function Nav({ user, theme, onThemeChange, onLogout, onMenuToggle, unreadCount }) {
+function Nav({ user, theme, onThemeChange, onLogout, onMenuToggle, unreadCount,
+              hubUnset }) {
   return html`
     <nav class="nav">
       <div class="nav-left">
@@ -590,7 +591,7 @@ function Nav({ user, theme, onThemeChange, onLogout, onMenuToggle, unreadCount }
         ${user ? html`
           <${UserMenu} user=${user} theme=${theme}
             onThemeChange=${onThemeChange} onLogout=${onLogout} />
-        ` : html`
+        ` : hubUnset ? null : html`
           <a class="nav-btn" href="#/login">${t('nav.login')}</a>
         `}
       </div>
@@ -681,18 +682,21 @@ function FirstRunPage({ onSet }) {
   };
 
   return html`
-    <div class="auth-page">
-      <h2>${t('firstrun.title')}</h2>
-      <p class="settings-hint">${t('firstrun.hint')}</p>
-      <form onSubmit=${submit}>
-        <input type="url" placeholder="https://meshbay.org" required
-          value=${url} onInput=${e => setUrl(e.target.value)} />
-        <button type="submit" disabled=${busy}>
-          ${busy ? '…' : t('firstrun.btn')}
-        </button>
-      </form>
-      ${error && html`<p class="error-msg">${error}</p>`}
-      <p class="settings-hint">${t('firstrun.note')}</p>
+    <div class="page-center">
+      <div class="card login-card">
+        <h2>${t('firstrun.title')}</h2>
+        <p class="settings-hint" style="margin-bottom:16px">${t('firstrun.hint')}</p>
+        <form onSubmit=${submit}>
+          <input type="text" placeholder="https://meshbay.org" required
+            autofocus value=${url}
+            onInput=${e => setUrl(e.target.value)} />
+          <button type="submit" disabled=${busy}>
+            ${busy ? t('firstrun.checking') : t('firstrun.btn')}
+          </button>
+        </form>
+        ${error && html`<div class="error-msg">${error}</div>`}
+        <p class="settings-hint" style="margin-top:16px">${t('firstrun.note')}</p>
+      </div>
     </div>
   `;
 }
@@ -1185,6 +1189,26 @@ const PIPELINE_WINDOW = 8;
  */
 async function _openDownloadTarget(filename, size = 0, pickerOpts = {},
                                    swSize = size) {
+  // On a desktop build this is the whole answer, and it comes first.
+  //
+  // The two browser paths below are both unavailable there — `showDirectoryPicker`
+  // does not exist, and Chromium refuses a service worker on a custom scheme —
+  // so without this the chain fell all the way through to its floor, which
+  // collects the file in the page and hands the browser a blob. A gigabyte of
+  // film meant a gigabyte of RAM, and a Save As dialog at the *end*.
+  if (platform.capabilities.nativeSave) {
+    try {
+      const native = await platform.nativeSave(
+        filename, { auto: downloads.getMode() === 'auto' });
+      // Null means the person dismissed the dialog, which is not an error and
+      // must not start a transfer.
+      return native || false;
+    } catch (err) {
+      console.warn('[MeshBay] native save failed:', platform.bridgeMessage(err));
+      return false;
+    }
+  }
+
   try {
     const target = await downloads.openTarget(filename);
     if (target) return target;
@@ -4013,10 +4037,20 @@ function SettingsPage({ user, theme, onThemeChange, groups }) {
     hubFetch('/v1/hub/version').then(setHubInfo).catch(() => {});
   }, []);
 
-  useEffect(() => { downloads.savedDirectory().then(setDlDir); }, []);
+  useEffect(() => {
+    // The desktop build remembers a path; the browser remembers a handle. Both
+    // answer "where do downloads go", and the row below renders either.
+    if (platform.folder.available) platform.folder.get().then(setDlDir);
+    else downloads.savedDirectory().then(setDlDir);
+  }, []);
 
   const pickFolder = useCallback(async () => {
     try {
+      if (platform.folder.available) {
+        const dir = await platform.folder.choose();
+        if (dir) setDlDir(dir);
+        return;
+      }
       const handle = await downloads.chooseDirectory();
       setDlDir(handle);
     } catch (err) {
@@ -4033,7 +4067,7 @@ function SettingsPage({ user, theme, onThemeChange, groups }) {
 
       <div class="settings-section">
         <h3 class="settings-heading">${t('settings.downloads')}</h3>
-        ${!downloads.SUPPORTED
+        ${!(downloads.SUPPORTED || platform.folder.available)
           ? html`<p class="settings-hint">${t('settings.dl_unsupported')}</p>`
           : html`
             <label class="settings-choice">
@@ -4054,7 +4088,8 @@ function SettingsPage({ user, theme, onThemeChange, groups }) {
             </label>
             <div class="settings-row" style="margin-top:10px">
               <span class="settings-label">
-                ${dlDir ? t('settings.dl_folder', { name: dlDir.name })
+                ${dlDir ? t('settings.dl_folder',
+                            { name: dlDir.name || String(dlDir) })
                         : t('settings.dl_no_folder')}
               </span>
               <span>
@@ -4063,7 +4098,8 @@ function SettingsPage({ user, theme, onThemeChange, groups }) {
                 </button>
                 ${dlDir && html`
                   <button class="btn-secondary" onClick=${async () => {
-                    await downloads.forgetDirectory();
+                    if (platform.folder.available) await platform.folder.forget();
+                    else await downloads.forgetDirectory();
                     setDlDir(null);
                   }}>${t('settings.dl_forget')}</button>
                 `}
@@ -4869,7 +4905,7 @@ function App() {
         onThemeChange=${changeTheme}
         onLogout=${authCtx.logout}
         onMenuToggle=${() => setMenuOpen(o => !o)}
-        unreadCount=${unreadCount} />
+        unreadCount=${unreadCount} hubUnset=${needsHub} />
       <div class="layout">
         ${user && html`<${Sidebar}
           groups=${groups}
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 ff7bd8a..e099edb 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/de.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/de.js
@@ -201,6 +201,7 @@ export default {
   'firstrun.title': 'Which hub?',
   'firstrun.hint': 'A hub holds your account and introduces you to nodes. It never sees your files, your messages or your keys.',
   'firstrun.btn': 'Continue',
+  'firstrun.checking': 'Checking…',
   'firstrun.note': 'There is no default on purpose: this application only trusts a hub for its API, never for the interface, which ships with the application itself.',
   'device.this_device': 'This device',
   'settings.keys_heading': 'Keys on this device',
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 738a862..a621084 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/en.js
@@ -193,6 +193,7 @@ export default {
   'firstrun.title': 'Which hub?',
   'firstrun.hint': 'A hub holds your account and introduces you to nodes. It never sees your files, your messages or your keys.',
   'firstrun.btn': 'Continue',
+  'firstrun.checking': 'Checking…',
   'firstrun.note': 'There is no default on purpose: this application only trusts a hub for its API, never for the interface, which ships with the application itself.',
   'device.this_device': 'This device',
   'settings.keys_heading': 'Keys on this device',
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 fb5cb0e..5fb4950 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/es.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/es.js
@@ -197,6 +197,7 @@ export default {
   'firstrun.title': 'Which hub?',
   'firstrun.hint': 'A hub holds your account and introduces you to nodes. It never sees your files, your messages or your keys.',
   'firstrun.btn': 'Continue',
+  'firstrun.checking': 'Checking…',
   'firstrun.note': 'There is no default on purpose: this application only trusts a hub for its API, never for the interface, which ships with the application itself.',
   'device.this_device': 'This device',
   'settings.keys_heading': 'Keys on this device',
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 a849730..c9480b6 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/fr.js
@@ -201,6 +201,7 @@ export default {
   'firstrun.title': 'Quel hub ?',
   'firstrun.hint': 'Un hub détient votre compte et vous met en relation avec les nœuds. Il ne voit ni vos fichiers, ni vos messages, ni vos clés.',
   'firstrun.btn': 'Continuer',
+  'firstrun.checking': 'Vérification…',
   'firstrun.note': 'Il n’y a volontairement pas de valeur par défaut : cette application ne fait confiance à un hub que pour son API, jamais pour l’interface, qui est livrée avec l’application.',
   'device.this_device': 'Cet appareil',
   'settings.keys_heading': 'Clés sur cet appareil',
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 17ad3e0..ac7c06c 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/it.js
@@ -200,6 +200,7 @@ export default {
   'firstrun.title': 'Which hub?',
   'firstrun.hint': 'A hub holds your account and introduces you to nodes. It never sees your files, your messages or your keys.',
   'firstrun.btn': 'Continue',
+  'firstrun.checking': 'Checking…',
   'firstrun.note': 'There is no default on purpose: this application only trusts a hub for its API, never for the interface, which ships with the application itself.',
   'device.this_device': 'This device',
   'settings.keys_heading': 'Keys on this device',
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 47221a6..b5d60ca 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/ja.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/ja.js
@@ -195,6 +195,7 @@ export default {
   'firstrun.title': 'Which hub?',
   'firstrun.hint': 'A hub holds your account and introduces you to nodes. It never sees your files, your messages or your keys.',
   'firstrun.btn': 'Continue',
+  'firstrun.checking': 'Checking…',
   'firstrun.note': 'There is no default on purpose: this application only trusts a hub for its API, never for the interface, which ships with the application itself.',
   'device.this_device': 'This device',
   'settings.keys_heading': 'Keys on this device',
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 617cdcd..304c90a 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/nl.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/nl.js
@@ -201,6 +201,7 @@ export default {
   'firstrun.title': 'Which hub?',
   'firstrun.hint': 'A hub holds your account and introduces you to nodes. It never sees your files, your messages or your keys.',
   'firstrun.btn': 'Continue',
+  'firstrun.checking': 'Checking…',
   'firstrun.note': 'There is no default on purpose: this application only trusts a hub for its API, never for the interface, which ships with the application itself.',
   'device.this_device': 'This device',
   'settings.keys_heading': 'Keys on this device',
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 ddbd1a9..98f36ce 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/locales/pl.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/locales/pl.js
@@ -207,6 +207,7 @@ export default {
   'firstrun.title': 'Which hub?',
   'firstrun.hint': 'A hub holds your account and introduces you to nodes. It never sees your files, your messages or your keys.',
   'firstrun.btn': 'Continue',
+  'firstrun.checking': 'Checking…',
   'firstrun.note': 'There is no default on purpose: this application only trusts a hub for its API, never for the interface, which ships with the application itself.',
   'device.this_device': 'This device',
   'settings.keys_heading': 'Keys on this device',
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 18c0921..5db5cd5 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
@@ -199,6 +199,7 @@ export default {
   'firstrun.title': 'Which hub?',
   'firstrun.hint': 'A hub holds your account and introduces you to nodes. It never sees your files, your messages or your keys.',
   'firstrun.btn': 'Continue',
+  'firstrun.checking': 'Checking…',
   'firstrun.note': 'There is no default on purpose: this application only trusts a hub for its API, never for the interface, which ships with the application itself.',
   'device.this_device': 'This device',
   'settings.keys_heading': 'Keys on this device',
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 d868335..b5f16fb 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
@@ -186,6 +186,7 @@ export default {
   'firstrun.title': 'Which hub?',
   'firstrun.hint': 'A hub holds your account and introduces you to nodes. It never sees your files, your messages or your keys.',
   'firstrun.btn': 'Continue',
+  'firstrun.checking': 'Checking…',
   'firstrun.note': 'There is no default on purpose: this application only trusts a hub for its API, never for the interface, which ships with the application itself.',
   'device.this_device': 'This device',
   'settings.keys_heading': 'Keys on this device',
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/platform.js b/packages/meshbay-hub/src/meshbay_hub/static/platform.js
index 994f558..372b666 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/platform.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/platform.js
@@ -176,13 +176,32 @@ export async function apiFetch(url, init) {
  * in `downloads.js`. Adding a native writer must not remove the three that
  * already work.
  */
-export async function nativeSave(suggestedName, size) {
+export async function nativeSave(suggestedName, { auto = true } = {}) {
   if (!bridge || !bridge.saveFile) return null;
-  return bridge.saveFile(suggestedName, size);
+  const sink = await bridge.saveFile(suggestedName, { auto });
+  if (!sink) return null;
+  // The shape every caller already expects from a download target: a `writable`
+  // with write/close/abort, and the name it was actually given on disk.
+  return {
+    name: sink.name,
+    writable: {
+      write: (bytes) => sink.write(bytes),
+      close: () => sink.close(),
+      abort: () => sink.abort(),
+    },
+  };
 }
 
+/** Where downloads go on a desktop build. Null in a browser. */
+export const folder = {
+  available: Boolean(bridge && bridge.folder),
+  async choose() { return bridge && bridge.folder ? bridge.folder.choose() : null; },
+  async get() { return bridge && bridge.folder ? bridge.folder.get() : null; },
+  async forget() { return bridge && bridge.folder ? bridge.folder.forget() : false; },
+};
+
 export default { isNative, hubBase, capabilities, secrets, nativeSave,
-                 apiFetch, device, bridgeMessage };
+                 apiFetch, device, bridgeMessage, folder };
 
 // Also a global, because `transport.js` is loaded as a classic script — it
 // predates the module graph and exposes `MeshBayTransport` the same way. The
@@ -191,5 +210,5 @@ export default { isNative, hubBase, capabilities, secrets, nativeSave,
 if (typeof window !== 'undefined') {
   window.MeshBayPlatform = { isNative, hubBase, capabilities, secrets,
                              nativeSave, apiFetch, device,
-                             bridgeMessage };
+                             bridgeMessage, folder };
 }
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/style.css b/packages/meshbay-hub/src/meshbay_hub/static/style.css
index 6b0b6fe..828e104 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/style.css
+++ b/packages/meshbay-hub/src/meshbay_hub/static/style.css
@@ -278,6 +278,14 @@ a:hover { text-decoration: underline; }
   padding: 24px;
 }
 
+/* A centring page already reserves the whole viewport below the header, so the
+   main column's own padding is added on top of it and the page overflows by
+   exactly that much — 48px, and a scrollbar on sign-in at every window size.
+   Measured, not deduced: `.page-center` ended at 795 and `main` at 819.
+   The page brings its own 24px, so dropping main's here changes nothing
+   visible and removes the duplicated arithmetic rather than adding more. */
+.main:has(> .page-center) { padding: 0; } 
+
 /* ── Cards ────────────────────────────────────────────────────────────────── */
 
 .card {