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/shared/interfaces/src/lib/settings.interface.ts

@@ -144,6 +144,11 @@ export interface Settings {
      * black bars. Off by default; only affects the built-in web players.
      */
     playerAmbientMode?: boolean;
+    /**
+     * Play a title's trailer, muted, behind the movie or series details a
+     * few seconds after the page opens. Off by default.
+     */
+    detailTrailerBackdrop?: boolean;
     /**
      * Dock the inline series player to the left and show an "Up Next"
      * episode rail in the leftover stage column on wide windows. On by