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.

YatteeTopShelf/TopShelfSnapshot.swift

@@ -0,0 +1,29 @@
+import Foundation
+
+// Must stay in sync with Yattee/Services/TopShelfSnapshot.swift.
+struct TopShelfItem: Codable, Hashable, Sendable {
+    let videoID: String
+    let title: String
+    let authorName: String
+    let duration: TimeInterval
+    let thumbnailURL: String?
+    let deepLinkURL: String
+    let progressSeconds: TimeInterval?
+}
+
+enum TopShelfSnapshot {
+    static func read(section: TopShelfSection, from defaults: UserDefaults = AppGroup.defaults) -> [TopShelfItem] {
+        guard let data = defaults.data(forKey: section.snapshotKey),
+              let items = try? JSONDecoder().decode([TopShelfItem].self, from: data) else {
+            return []
+        }
+        return items
+    }
+
+    static func enabledSections(from defaults: UserDefaults = AppGroup.defaults) -> [TopShelfSection] {
+        guard let raw = defaults.array(forKey: AppGroup.enabledSectionsKey) as? [String] else {
+            return TopShelfSection.defaultOrder
+        }
+        return raw.compactMap { TopShelfSection(rawValue: $0) }
+    }
+}