fix(app_controller): async RAG engine init to avoid blocking startup

RAG engine initialization (including chromadb import and index loading)
now happens in a background thread, allowing the GUI to show immediately.
The app was blocking for 5+ seconds during init_state() because RAG was
enabled in config. Now RAG loads asynchronously.

src/app_controller.py

@@ -1234,10 +1234,12 @@ class AppController:
 
   self.rag_engine = None
   if self.rag_config.enabled:
+   def _init_rag_engine():
     from src import rag_engine
     self.rag_engine = rag_engine.RAGEngine(self.rag_config, self.active_project_root)
     if self.rag_engine.is_empty():
      self._rebuild_rag_index()
+   threading.Thread(target=_init_rag_engine, daemon=True).start()
 
   from src.personas import PersonaManager
   self.persona_manager = PersonaManager(Path(self.active_project_path).parent if self.active_project_path else None)