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/portal/shared/data-access/src/lib/collection/unified-favorites-data.service.ts

@@ -807,12 +807,17 @@ export class UnifiedFavoritesDataService {
     }
 
     private buildXtreamPositionUpdates(items: UnifiedCollectionItem[]) {
-        const updates: { content_id: number; position: number }[] = [];
+        const updates: {
+            content_id: number;
+            playlist_id: string;
+            position: number;
+        }[] = [];
 
         for (const item of items) {
             if (item.sourceType === 'xtream' && item.contentId != null) {
                 updates.push({
                     content_id: item.contentId,
+                    playlist_id: item.playlistId,
                     position: updates.length,
                 });
             }

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;
 }