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/src/rust/api.dart

@@ -226,6 +226,32 @@ String exportDiagnostics() => RustLib.instance.api.crateApiExportDiagnostics();
 Future<void> initStorage({required String dir}) =>
     RustLib.instance.api.crateApiInitStorage(dir: dir);
 
+/// Wire the blob cache to a platform-private cache directory.
+Future<void> initCache({required String dir}) {
+  final api = RustLib.instance.api as dynamic;
+  try {
+    return api.crateApiInitCache(dir: dir) as Future<void>;
+  } on NoSuchMethodError {
+    return Future.value();
+  }
+}
+
+/// Return an avatar from cache when present; otherwise download it.
+Future<List<int>?> downloadAvatar({
+  required String avatarHash,
+  required String clientUid,
+}) {
+  final api = RustLib.instance.api as dynamic;
+  try {
+    return api.crateApiDownloadAvatar(
+      avatarHash: avatarHash,
+      clientUid: clientUid,
+    ) as Future<List<int>?>;
+  } on NoSuchMethodError {
+    return Future.value();
+  }
+}
+
 /// List persisted bookmarks.
 Future<List<BridgeBookmark>> listBookmarks() =>
     RustLib.instance.api.crateApiListBookmarks();