feat(core,bridge): add get_icon with coalescing and negative cache

- FileTransferService::get_icon() mirrors get_avatar pattern
- ChanoraSession::get_icon() delegates through FileTransferService
- Bridge download_icon() exposed for Flutter
- Dart downloadIcon() shim added
- Uses PREFIX_ICON (ic_<crc32u>) cache key format
- 1 new unit test (cached icon hit)

apps/chanora_flutter/lib/src/rust/api.dart

@@ -252,6 +252,15 @@ Future<List<int>?> downloadAvatar({
   }
 }
 
+Future<List<int>?> downloadIcon({required BigInt iconId}) {
+  final api = RustLib.instance.api as dynamic;
+  try {
+    return api.crateApiDownloadIcon(iconId: iconId) as Future<List<int>?>;
+  } on NoSuchMethodError {
+    return Future.value();
+  }
+}
+
 /// List persisted bookmarks.
 Future<List<BridgeBookmark>> listBookmarks() =>
     RustLib.instance.api.crateApiListBookmarks();