feat(portals): optional trailer backdrop behind the details hero

Settings → Playback → "Play trailers in details background" (off by
default) hands the trailer embed URL to the detail shell; the new
app-hero-trailer-backdrop plays it muted and looping under the scrim
after three idle seconds with a corner mute toggle. It never starts under
prefers-reduced-motion or with saveData, and stops while the hero is off
screen or the window is unfocused. Xtream and Stalker movies and series
are wired; M3U has no trailer source.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

libs/workspace/shell/util/src/lib/settings-search/settings-search-entries-playback.ts

@@ -44,6 +44,14 @@ export const PLAYBACK_SETTINGS_SEARCH_ENTRIES: readonly SettingsSearchEntry[] =
             keywords: ['ambient', 'glow', 'backlight'],
             fallbackId: 'video-player',
         },
+        {
+            id: 'detail-trailer-backdrop',
+            section: 'playback',
+            labelKey: 'SETTINGS.PLAY_TRAILERS_IN_BACKGROUND',
+            descriptionKey: 'SETTINGS.PLAY_TRAILERS_IN_BACKGROUND_DESCRIPTION',
+            keywords: ['trailer', 'youtube', 'backdrop', 'details'],
+            fallbackId: 'video-player',
+        },
         {
             id: 'player-up-next-rail',
             section: 'playback',