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 {

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) {

flutter_client/lib/theme/theme_extension.dart

@@ -30,7 +30,7 @@ class FabledSwordTheme extends ThemeExtension<FabledSwordTheme> {
   final Color warning, error, info;
   final TextStyle display, body, mono;
 
-  factory FabledSwordTheme.dark() => FabledSwordTheme(
+  factory FabledSwordTheme.dark() => const FabledSwordTheme(
         accent: FabledSwordFlatTokens.accent,
         obsidian: FabledSwordDarkTokens.obsidian,
         iron: FabledSwordDarkTokens.iron,
@@ -50,7 +50,7 @@ class FabledSwordTheme extends ThemeExtension<FabledSwordTheme> {
         mono: TextStyle(fontFamily: FabledSwordFlatTokens.fontMono),
       );
 
-  factory FabledSwordTheme.light() => FabledSwordTheme(
+  factory FabledSwordTheme.light() => const FabledSwordTheme(
         accent: FabledSwordFlatTokens.accent,
         obsidian: FabledSwordLightTokens.obsidian,
         iron: FabledSwordLightTokens.iron,

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);
   });