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/settings/settings_screen.dart

@@ -503,26 +503,32 @@ class _AppearanceSection extends ConsumerWidget {
   Widget build(BuildContext context, WidgetRef ref) {
     final fs = Theme.of(context).extension<FabledSwordTheme>()!;
     final current = ref.watch(themeModeProvider).value ?? AppThemeMode.system;
-    return Column(crossAxisAlignment: CrossAxisAlignment.start, children: [
-      const _SectionHeader('Appearance'),
-      for (final mode in AppThemeMode.values)
-        RadioListTile<AppThemeMode>(
-          key: Key('appearance_${mode.name}'),
-          title: Text(_label(mode), style: TextStyle(color: fs.parchment)),
-          subtitle: mode == AppThemeMode.system
-              ? Text('Match the device setting',
-                  style: TextStyle(color: fs.ash))
-              : null,
-          value: mode,
-          groupValue: current,
-          activeColor: fs.accent,
-          onChanged: (m) {
-            if (m != null) {
-              ref.read(themeModeProvider.notifier).set(m);
-            }
-          },
-        ),
-    ]);
+    // RadioGroup is the post-Flutter-3.32 API: the ancestor owns
+    // groupValue/onChanged so individual RadioListTiles don't have
+    // to repeat them. Pre-3.32 RadioListTile.groupValue/onChanged
+    // are deprecated.
+    return RadioGroup<AppThemeMode>(
+      groupValue: current,
+      onChanged: (m) {
+        if (m != null) {
+          ref.read(themeModeProvider.notifier).set(m);
+        }
+      },
+      child: Column(crossAxisAlignment: CrossAxisAlignment.start, children: [
+        const _SectionHeader('Appearance'),
+        for (final mode in AppThemeMode.values)
+          RadioListTile<AppThemeMode>(
+            key: Key('appearance_${mode.name}'),
+            title: Text(_label(mode), style: TextStyle(color: fs.parchment)),
+            subtitle: mode == AppThemeMode.system
+                ? Text('Match the device setting',
+                    style: TextStyle(color: fs.ash))
+                : null,
+            value: mode,
+            activeColor: fs.accent,
+          ),
+      ]),
+    );
   }
 
   String _label(AppThemeMode m) => switch (m) {