fix(project): Non-blocking project switch with stale-ui tint

When switching projects, the previous implementation ran the entire
save/load/refresh sequence on the main thread. With large project files
or slow disks, this caused the UI to freeze for several seconds.

Fix:
- _switch_project now returns immediately after setting flags; the
  actual work runs in a daemon thread (_do_project_switch)
- New is_project_stale() property returns True while a switch is queued
  or running; the GUI renders an amber/yellow tint overlay to signal
  the controller state lags the user's last click
- AI ops are gated: _api_generate returns HTTP 409, _handle_generate_send
  and _handle_md_only early-return with ai_status feedback, all when
  is_project_stale() is true
- Queued switches (clicking project A then B in rapid succession) are
  coalesced: B replaces A as the target; once A completes, B is
  triggered automatically via the finally branch in _do_project_switch
- New state fields: _project_switch_in_progress, _project_switch_pending_path,
  _project_switch_thread, _project_switch_lock
- AppController state class attributes use hasattr guard for _app to
  keep the controller usable standalone in tests/headless mode

UX:
- Render loop keeps drawing during the switch
- User can still scroll, switch tabs, browse files
- Amber tint + popup explains what's happening and that AI ops are paused
- ai_status shows the target project name

Tests:
- _wait_for_switch helper added for the new async switch flow
- All 7 existing switch tests updated to call _wait_for_switch
- 2 new tests:
  - test_switch_project_non_blocking: verifies _switch_project returns
    in <0.2s and is_project_stale() is True during the switch
  - test_api_generate_blocked_while_stale: verifies _api_generate
    raises HTTPException(409) while a switch is in progress

All 33 related tests pass.

src/app_controller.py

@@ -272,6 +272,8 @@ def _api_generate(controller: 'AppController', req: GenerateRequest) -> dict[str
  """
  if not req.prompt.strip():
   raise HTTPException(status_code=400, detail="Prompt cannot be empty")
+ if controller.is_project_stale():
+  raise HTTPException(status_code=409, detail="Project switch in progress; AI ops are disabled until it completes.")
  with controller._send_thread_lock:
   start_time = time.time()
   try:
@@ -800,6 +802,7 @@ class AppController:
   self._pending_dialog_lock: threading.Lock = threading.Lock()
   self._api_event_queue_lock: threading.Lock = threading.Lock()
   self._rag_engine_lock: threading.Lock = threading.Lock()
+  self._project_switch_lock: threading.Lock = threading.Lock()
 
   # --- Internal State ---
   self._ai_status: str = "idle"
@@ -842,6 +845,9 @@ class AppController:
   self._pending_mma_approvals: List[Dict[str, Any]] = []
   self._mma_approval_open: bool = False
   self._mma_approval_edit_mode: bool = False
+  self._project_switch_in_progress: bool = False
+  self._project_switch_pending_path: Optional[str] = None
+  self._project_switch_thread: Optional[threading.Thread] = None
   self._mma_approval_payload: str = ""
   self._pending_mma_spawns: List[Dict[str, Any]] = []
   self._mma_spawn_open: bool = False
@@ -2683,31 +2689,78 @@ class AppController:
   file_items_as_dicts = [{"path": f.path if hasattr(f, "path") else str(f)} for f in self.files]
   mcp_client.configure(file_items_as_dicts, [str(project_root)])
 
+ def is_project_stale(self) -> bool:
+  """True when a project switch is queued or running; UI should tint
+  to signal the controller state lags the user's last click."""
+  with self._project_switch_lock:
+   if self._project_switch_in_progress:
+    return True
+   pending = self._project_switch_pending_path
+  if pending and pending != self.active_project_path:
+   return True
+  return False
+
  def _switch_project(self, path: str) -> None:
   """
     [C: src/gui_2.py:App._render_projects_panel]
+  
+  Non-blocking: returns immediately, marks the controller as stale,
+  and runs the actual save/load work in a background thread so the
+  render loop keeps drawing and lightweight UI interactions (scrolling,
+  selecting tabs) remain responsive.
   """
-  if path == self.active_project_path:
+  if path == self.active_project_path and not self.is_project_stale():
    return
   if not Path(path).exists():
    self.ai_status = f"project file not found: {path}"
    return
-  self._flush_to_project()
+  with self._project_switch_lock:
+   if self._project_switch_in_progress:
+    if self._project_switch_pending_path == path:
+     return
+    self._project_switch_pending_path = path
+    self.ai_status = f"switch queued: {Path(path).stem} (waiting on {Path(self._project_switch_pending_path or '').stem})"
+    return
+   self._project_switch_in_progress = True
+   self._project_switch_pending_path = path
+  self.ai_status = f"switching to: {Path(path).stem} (stale ui - ops disabled)"
+  self._project_switch_thread = threading.Thread(
+   target=self._do_project_switch,
+   args=(path,),
+   daemon=True,
+  )
+  self._project_switch_thread.start()
+
+ def _do_project_switch(self, path: str) -> None:
   try:
-   self.project = project_manager.load_project(path)
-   self.active_project_path = path
-   new_root = Path(path).parent
-   self.preset_manager = presets.PresetManager(new_root)
-   self.tool_preset_manager = tool_presets.ToolPresetManager(new_root)
-   from src.personas import PersonaManager
-   self.persona_manager = PersonaManager(new_root)
-  except Exception as e:
-   self.ai_status = f"failed to load project: {e}"
-   return
-  self._refresh_from_project()
-  file_items_as_dicts = [{"path": f.path if hasattr(f, "path") else str(f)} for f in self.files]
-  mcp_client.configure(file_items_as_dicts, [str(new_root)])
-  self.ai_status = f"switched to: {Path(path).stem}"
+   self._flush_to_project()
+   try:
+    new_project = project_manager.load_project(path)
+   except Exception as e:
+    self.ai_status = f"failed to load project: {e}"
+    return
+   try:
+    self.project = new_project
+    self.active_project_path = path
+    new_root = Path(path).parent
+    self.preset_manager = presets.PresetManager(new_root)
+    self.tool_preset_manager = tool_presets.ToolPresetManager(new_root)
+    from src.personas import PersonaManager
+    self.persona_manager = PersonaManager(new_root)
+   except Exception as e:
+    self.ai_status = f"failed to init managers: {e}"
+    return
+   self._refresh_from_project()
+   file_items_as_dicts = [{"path": f.path if hasattr(f, "path") else str(f)} for f in self.files]
+   mcp_client.configure(file_items_as_dicts, [str(new_root)])
+   self.ai_status = f"switched to: {Path(path).stem}"
+  finally:
+   with self._project_switch_lock:
+    pending = self._project_switch_pending_path
+    self._project_switch_in_progress = False
+    self._project_switch_pending_path = None
+   if pending and pending != self.active_project_path and Path(pending).exists():
+    self._switch_project(pending)
 
  def _refresh_from_project(self) -> None:
   # Deserialize FileItems in files.paths
@@ -3279,6 +3332,9 @@ class AppController:
      Logic for the 'MD Only' action.
     [C: src/gui_2.py:App._render_message_panel]
   """
+  if self.is_project_stale():
+   self.ai_status = "project switch in progress; MD generation disabled"
+   return
 
   def worker():
    """
@@ -3324,6 +3380,9 @@ class AppController:
      Logic for the 'Gen + Send' action.
     [C: src/gui_2.py:App._render_message_panel, src/gui_2.py:App._render_synthesis_panel, src/gui_2.py:App._render_takes_panel, tests/test_gui_events_v2.py:test_handle_generate_send_pushes_event, tests/test_symbol_parsing.py:test_handle_generate_send_appends_definitions, tests/test_symbol_parsing.py:test_handle_generate_send_no_symbols]
   """
+  if self.is_project_stale():
+   self.ai_status = "project switch in progress; AI ops disabled"
+   return
 
   def worker():
    """