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/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',
     );
   }