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/Info.plist

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>NSExtension</key>
+	<dict>
+		<key>NSExtensionPointIdentifier</key>
+		<string>com.apple.tv-top-shelf</string>
+		<key>NSExtensionPrincipalClass</key>
+		<string>$(PRODUCT_MODULE_NAME).TopShelfContentProvider</string>
+	</dict>
+</dict>
+</plist>