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/AppGroup.swift

@@ -0,0 +1,12 @@
+import Foundation
+
+enum AppGroup {
+    static let identifier = "group.stream.yattee.app.shared"
+
+    /// UserDefaults key holding an ordered [String] of enabled TopShelfSection raw values.
+    static let enabledSectionsKey = "topShelf.enabledSections"
+
+    static var defaults: UserDefaults {
+        UserDefaults(suiteName: identifier) ?? .standard
+    }
+}