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
