feat(flutter): add cache initialization wiring and avatar download shims

- Add wireCache() to app_bootstrap using getApplicationCacheDirectory()
- Call wireCache() after wireStorage() in main bootstrap flow
- Add Dart-side initCache and downloadAvatar wrapper shims in api.dart
- Update Cargo.lock for new chanora_cache dependency

apps/chanora_flutter/lib/main.dart

@@ -93,7 +93,10 @@ String _kAppVersion = appSemverBaseline;
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
   await RustLib.init();
-  unawaited(wireStorage());
+  unawaited(() async {
+    await wireStorage();
+    await wireCache();
+  }());
   unawaited(wireConnectivity());
   wireAudioLifecycle();
   await configureBundledVadModels();