fix(app_controller): lazy load rag_engine to avoid blocking startup

Before this change, app_controller imported rag_engine at module level which
pulled in chromadb (~0.45s). Now rag_engine is only imported when RAG is
actually enabled and needed. This improves startup time significantly.

src/app_controller.py

@@ -28,7 +28,6 @@ from src import orchestrator_pm
 from src import paths
 from src import performance_monitor
 from src import project_manager
-from src import rag_engine
 from src import session_logger
 from src import workspace_manager
 from src import presets
@@ -225,7 +224,7 @@ class AppController:
   self.mcp_config: models.MCPConfiguration = models.MCPConfiguration()
   self.view_presets: list[models.NamedViewPreset] = []
   self.rag_config: Optional[models.RAGConfig] = None
-  self.rag_engine: Optional[rag_engine.RAGEngine] = None
+  self.rag_engine: Optional[Any] = None
   self.rag_status: str = 'idle'
   # AI settings state
   self._current_provider: str = "gemini"
@@ -571,6 +570,7 @@ class AppController:
  def rag_enabled(self, value: bool) -> None:
   if self.rag_config:
    self.rag_config.enabled = value
+   from src import rag_engine
    self.rag_engine = rag_engine.RAGEngine(self.rag_config, self.active_project_root)
 
  @property
@@ -580,6 +580,7 @@ class AppController:
  def rag_source(self, value: str) -> None:
   if self.rag_config:
    self.rag_config.vector_store.provider = value
+   from src import rag_engine
    if self.rag_engine: self.rag_engine = rag_engine.RAGEngine(self.rag_config, self.active_project_root)
 
  @property
@@ -589,6 +590,7 @@ class AppController:
  def rag_emb_provider(self, value: str) -> None:
   if self.rag_config:
    self.rag_config.embedding_provider = value
+   from src import rag_engine
    if self.rag_engine: self.rag_engine = rag_engine.RAGEngine(self.rag_config, self.active_project_root)
 
  @property
@@ -1230,9 +1232,12 @@ class AppController:
   else:
    self.rag_config = models.RAGConfig()
 
-  self.rag_engine = rag_engine.RAGEngine(self.rag_config, self.active_project_root)
-  if self.rag_config.enabled and self.rag_engine.is_empty():
-   self._rebuild_rag_index()
+  self.rag_engine = None
+  if self.rag_config.enabled:
+   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()
 
   from src.personas import PersonaManager
   self.persona_manager = PersonaManager(Path(self.active_project_path).parent if self.active_project_path else None)