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/Settings/SettingsKey.swift

@@ -65,6 +65,9 @@ enum SettingsKey: String, CaseIterable {
     case homeSectionItemsLimit
     case homeSectionLayout
 
+    // Top Shelf (tvOS)
+    case topShelfSections
+
     // Tab Bar (compact size class)
     case tabBarItemOrder
     case tabBarItemVisibility
@@ -126,6 +129,8 @@ enum SettingsKey: String, CaseIterable {
              // Home layout — different UI paradigms per platform
              .homeShortcutOrder, .homeShortcutVisibility, .homeShortcutLayout,
              .homeSectionOrder, .homeSectionVisibility, .homeSectionItemsLimit, .homeSectionLayout,
+             // Top Shelf — tvOS only
+             .topShelfSections,
              // Tab bar (compact size class) layout
              .tabBarItemOrder, .tabBarItemVisibility, .tabBarStartupTab,
              // Sidebar layout/selection