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/Views/Settings/SettingsView.swift

@@ -131,6 +131,10 @@ struct SettingsView: View {
                         Label(String(localized: "settings.layoutNavigation.title"), systemImage: "hand.tap")
                     }
 
+                    NavigationLink { TVSidebarDetailContainer(systemImage: "rectangle.on.rectangle.angled", title: String(localized: "settings.topShelf.title", defaultValue: "Top Shelf")) { TopShelfSettingsView() } } label: {
+                        Label(String(localized: "settings.topShelf.title", defaultValue: "Top Shelf"), systemImage: "rectangle.on.rectangle.angled")
+                    }
+
                     NavigationLink { TVSidebarDetailContainer(systemImage: "play.circle", title: String(localized: "settings.playback.sectionTitle")) { PlaybackSettingsView() } } label: {
                         Label(String(localized: "settings.playback.sectionTitle"), systemImage: "play.circle")
                     }