feat: add itchat-uos WeChat adapter with QR code login

- Add itchat-uos adapter supporting personal WeChat via QR code login
- Implement message/event converters for text, image, voice, sharing types
- Bridge sync itchat callbacks to async LangBot pipeline via asyncio
- Add QR login API endpoints with session management
- Add frontend QR code login dialog integration
- Fix plugin connector handler attribute check
- Use fresh Core instance per login to avoid singleton state pollution

src/langbot/pkg/plugin/connector.py

@@ -736,7 +736,7 @@ class PluginRuntimeConnector(ManagedRuntimeConnector):
     ) -> context.EventContext:
         event_ctx = context.EventContext.from_event(event)
 
-        if not self.is_enable_plugin:
+        if not self.is_enable_plugin or not hasattr(self, 'handler'):
             event_ctx._emitted_plugins = []
             event_ctx._response_sources = []
             return event_ctx