fix(favorites): scope reorder position writes by playlist

The global favorites reorder wrote the new position filtering only by
content_id, so two Xtream playlists holding a favorite with the same
content_id would clobber each other's persisted order (greptile P1).

Thread playlist_id through the whole reorder path — the renderer builder
(UnifiedCollectionItem already carries playlistId), the IPC contract
(ElectronBridgeFavoriteReorderUpdate + inline payload types), the worker
op — and scope the prepared UPDATE by (contentId, playlistId), matching
the favorites composite unique index.

Tests: favorites.operations.spec asserts the playlistId placeholder and
per-row playlistId payload; preload contract fixture updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

libs/shared/interfaces/src/lib/electron-api.interface.ts

@@ -414,6 +414,8 @@ export interface ElectronBridgeGlobalRecentlyAddedItem extends ElectronBridgeXtr
 
 export interface ElectronBridgeFavoriteReorderUpdate {
     content_id: number;
+    /** Favorites are playlist-scoped — scope the position write per playlist */
+    playlist_id: string;
     position: number;
 }