Add tvOS Top Shelf extension

Surfaces Continue Watching, Recent Feed, and Recent Bookmarks in the
Apple TV Home top shelf when Yattee is focused. Tapping a tile opens
the video via the existing yattee://video/{id} deep link.

- New YatteeTopShelf app extension target (tvOS only). LD_ENTRY_POINT is
  overridden to _NSExtensionMain; the tv-app-extension product type
  defaults to _TVExtensionMain which is for the pre-tvOS-13 legacy API
  and crashes modern TVTopShelfContentProvider subclasses at launch.
- Main app writes per-section JSON snapshots (capped at 10 items each)
  to a shared App Group UserDefaults suite after bookmark, watch-history,
  and feed-cache changes, plus an initial write on launch.
- Enabled-sections list is mirrored to the same App Group so the
  extension can respect the user's selection without touching SwiftData.
- Settings → Top Shelf (tvOS only) lets the user toggle sections.
- Deep link playback shows a loading toast while video details are
  fetched, and an error toast if no source is configured.

Yattee/Data/DataManager+Bookmarks.swift

@@ -140,6 +140,7 @@ extension DataManager {
             modelContext.insert(bookmark)
             save()
         }
+        TopShelfSnapshotWriter.writeBookmarks(dataManager: self)
     }
     
     /// Updates bookmark tags and note for a video.

Yattee/Data/DataManager+WatchHistory.swift

@@ -69,6 +69,7 @@ extension DataManager {
 
             // Queue for CloudKit sync
             cloudKitSync?.queueWatchEntrySave(entry)
+            TopShelfSnapshotWriter.writeContinueWatching(dataManager: self)
         } catch {
             LoggingService.shared.logCloudKitError("Failed to update watch progress", error: error)
         }
@@ -148,6 +149,7 @@ extension DataManager {
             modelContext.insert(watchEntry)
             save()
         }
+        TopShelfSnapshotWriter.writeContinueWatching(dataManager: self)
     }
 
     /// Clears all watch history.