feat(api_hooks): add /api/project_switch_status endpoint for deterministic test signaling

Adds a new endpoint that exposes the project-switch state machine so tests
can poll for completion instead of guessing with timeouts.

- AppController: track _project_switch_error on failure paths
- src/api_hooks.py: GET /api/project_switch_status returns
  {in_progress, pending_path, active_path, error}
- src/api_hook_client.py: get_project_switch_status() helper
- tests/test_api_hooks_project_switch.py: 3 unit tests for client + endpoint
  shape, 1 live_gui test for the default-idle case

src/app_controller.py

@@ -803,6 +803,7 @@ class AppController:
   self._project_switch_lock:       threading.Lock = threading.Lock()
   self._project_switch_in_progress: bool = False
   self._project_switch_pending_path:  Optional[str] = None
+  self._project_switch_error:         Optional[str] = None
   # --- Shared background pool + proactive warmup (startup_speedup_20260606) ---
   self._io_pool = make_io_pool()
   _install_sigint_exit_handler(self)
@@ -2690,12 +2691,14 @@ class AppController:
   self.ai_status = "config saved"
 
  def _do_project_switch(self, path: str) -> None:
+  self._project_switch_error = None
   try:
    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}"
+    self._project_switch_error = f"load failed: {e}"
     return
    try:
     self.project = new_project
@@ -2707,6 +2710,7 @@ class AppController:
     self.persona_manager = PersonaManager(new_root)
    except Exception as e:
     self.ai_status = f"failed to init managers: {e}"
+    self._project_switch_error = f"manager init failed: {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]