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/Core/SettingsManager.swift

@@ -109,6 +109,9 @@ final class SettingsManager {
     var _homeSectionItemsLimit: Int?
     var _homeSectionLayout: HomeSectionLayout?
 
+    // Top Shelf (tvOS)
+    var _topShelfSections: [TopShelfSection]?
+
     // Tab bar settings (compact size class only - iOS)
     var _tabBarItemOrder: [TabBarItem]?
     var _tabBarItemVisibility: [TabBarItem: Bool]?
@@ -453,6 +456,7 @@ final class SettingsManager {
         _homeSectionVisibility = nil
         _homeSectionItemsLimit = nil
         _homeSectionLayout = nil
+        _topShelfSections = nil
         _tabBarItemOrder = nil
         _tabBarItemVisibility = nil
         _sidebarMainItemOrder = nil