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/test/settings/appearance_section_test.dart

@@ -49,11 +49,15 @@ void main() {
     expect(find.byKey(const Key('appearance_light')), findsOneWidget);
     expect(find.byKey(const Key('appearance_dark')), findsOneWidget);
 
-    // The dark radio should be selected.
+    // The RadioGroup ancestor owns groupValue post-Flutter-3.32; the
+    // selected mode is read off it rather than off individual tiles.
+    final group = t.widget<RadioGroup<AppThemeMode>>(
+      find.byType(RadioGroup<AppThemeMode>),
+    );
+    expect(group.groupValue, AppThemeMode.dark);
     final darkRadio = t.widget<RadioListTile<AppThemeMode>>(
       find.byKey(const Key('appearance_dark')),
     );
-    expect(darkRadio.groupValue, AppThemeMode.dark);
     expect(darkRadio.value, AppThemeMode.dark);
   });