feat(node,hub): add Videos group app (poster grid, flat list, TMDB metadata)

Implements docs/mediacenter.md: a "Videos" group application built on the
existing files index rather than a separate catalogue. On the node side,
new indexer enrichment (technical probe, filename/season parsing, thumbnail
generation) runs per-file once an operator has chosen a video_root for the
group, plus a TMDB client for on-demand poster/metadata lookups (never
client-side, thumbnails delivered over the existing chunk path). On the hub
side, a new video-app.js renders a lazily-mounted poster grid or a
thumbnail-only flat list, with TMDB entirely optional per group.

Along the way: the global apps registry now drives Settings' default-tab
picker instead of a hardcoded list, and the video_root is configured from
group Settings (like uploads) rather than from Files, with the node
refusing to run any TMDB/thumbnail work until one is set.

Fixes several bugs found via live testing against a real library, notably
a race between two effects writing the same "image ready" state that could
leave a poster grid spinning forever on a same-tab revisit — see
mediacenter.md §5.4 for the full account of each one.

1 files changed, 2 insertions, 2 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/app.js b/packages/meshbay-hub/src/meshbay_hub/static/app.js
index 044457b..58ffe8e 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/app.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/app.js
@@ -16,6 +16,7 @@ import {
   refreshAccessToken,
 } from './hub-client.js';
 import { GroupPage } from './group-page.js';
+import { APPS } from './apps.js';
 
 // ── Constants ────────────────────────────────────────────────────────────────
 
@@ -1731,8 +1732,7 @@ function SettingsPage({ user, theme, onThemeChange, groups, onPrefsChange }) {
           <span class="settings-label">${t('settings.default_tab')}</span>
           <select class="settings-select" value=${defaultTab}
             onChange=${changeDefaultTab}>
-            <option value="chat">${t('group.tab_chat')}</option>
-            <option value="files">${t('group.tab_files')}</option>
+            ${APPS.map((a) => html`<option key=${a.key} value=${a.key}>${t(a.labelKey)}</option>`)}
             <option value="settings">${t('group.tab_settings')}</option>
           </select>
         </div>