fix(flutter): clear analyze --fatal-infos issues

Three classes:

1. album_cover_cache.dart docstring used <applicationCacheDirectory>
   and <albumId> which the analyzer reads as unintended HTML. Wrapped
   the path in backticks and used {} placeholders.

2. settings AppearanceSection used RadioListTile.groupValue + onChanged,
   deprecated in Flutter 3.32+. Wrapped the radios in a RadioGroup
   ancestor (the new API) so individual tiles only declare value +
   activeColor. Test updated to read groupValue off the RadioGroup
   ancestor.

3. theme_extension.dart factories built FabledSwordTheme(...) without
   const, even though all args are static const tokens. Added const to
   the outer constructor calls in both .dark() and .light() — this
   propagates const to the inner TextStyle(...) calls automatically.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/player/album_cover_cache.dart

@@ -10,7 +10,7 @@ import 'package:path_provider/path_provider.dart';
 /// and doesn't carry our Bearer header, so we pre-fetch via the
 /// authenticated dio and hand the system a local file path instead.
 ///
-/// Cache layout: <applicationCacheDirectory>/album_covers/<albumId>.jpg.
+/// Cache layout: `{applicationCacheDirectory}/album_covers/{albumId}.jpg`.
 /// No explicit eviction — covers are tiny and OS clears app cache when
 /// space is tight.
 class AlbumCoverCache {