feat: retire built-in download providers, add isolated extension runtimes, Google Sans Flex font, and monochrome icon support

- Remove all built-in download provider code paths (DownloadTrack, DownloadWithFallback, tryBuiltInProvider, isBuiltInDownloadProvider, normalizeQualityForBuiltIn)
- Simplify DownloadByStrategy to route exclusively through extension providers
- Add newIsolatedExtensionRuntime() for concurrent per-download Goja VMs
- Extract reusable initializeExtensionRuntimeWithSettings() and runCleanupOnVM()
- Add TestExtensionDownloadUsesIsolatedRuntimeForConcurrentCalls
- Add Google Sans Flex font family to app themes
- Add Android adaptive icon monochrome support
- Regenerate iOS and Android app icons

lib/screens/settings/options_settings_page.dart

@@ -15,8 +15,6 @@ class OptionsSettingsPage extends ConsumerWidget {
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final settings = ref.watch(settingsProvider);
-    final extensionState = ref.watch(extensionProvider);
-    final hasExtensions = extensionState.extensions.isNotEmpty;
     final colorScheme = Theme.of(context).colorScheme;
     final topPadding = normalizedHeaderTopPadding(context);
 
@@ -93,18 +91,6 @@ class OptionsSettingsPage extends ConsumerWidget {
                     onChanged: (v) =>
                         ref.read(settingsProvider.notifier).setAutoFallback(v),
                   ),
-                  if (hasExtensions)
-                    SettingsSwitchItem(
-                      icon: Icons.extension,
-                      title: context.l10n.optionsUseExtensionProviders,
-                      subtitle: settings.useExtensionProviders
-                          ? context.l10n.optionsUseExtensionProvidersOn
-                          : context.l10n.optionsUseExtensionProvidersOff,
-                      value: settings.useExtensionProviders,
-                      onChanged: (v) => ref
-                          .read(settingsProvider.notifier)
-                          .setUseExtensionProviders(v),
-                    ),
                   SettingsSwitchItem(
                     icon: Icons.sell_outlined,
                     title: context.l10n.optionsEmbedMetadata,

lib/theme/app_theme.dart

@@ -32,6 +32,7 @@ class AppTheme {
       switchTheme: _switchTheme(scheme),
       chipTheme: _chipTheme(scheme),
       dividerTheme: _dividerTheme(scheme),
+      fontFamily: 'Google Sans Flex',
     );
   }
 
@@ -67,6 +68,7 @@ class AppTheme {
       switchTheme: _switchTheme(scheme),
       chipTheme: _chipTheme(scheme),
       dividerTheme: _dividerTheme(scheme),
+      fontFamily: 'Google Sans Flex',
     );
   }