feat(backend): introduce session controller and status models

Implement DeviceController and SessionController to bridge QML requests with backend state, supporting playback replay tracking, cluster averaging, and slot error boundaries.

src/pygui/backend/controllers/session_controller.py

@@ -87,12 +87,7 @@ class SessionController(QObject):
 
     def _load_settings(self, settings: dict) -> None:
         """Restore session state from persisted settings dict."""
-        last_file = settings.get("session_last_file", "")
-        if last_file:
-            try:
-                self.loadFile(last_file)
-            except Exception:
-                log.warning("Could not restore last session file: %s", last_file)
+        # Do NOT auto-restore the last session file — always start fresh.
 
     def collect_settings(self) -> dict:
         """Return a dict of session state to persist."""