Fix sidecar pipe crash on state changes and show logged-in state in settings

The state callback in main_headless.py wrote events to stdout
synchronously, so an EINVAL on the Tauri sidecar pipe (Windows) bubbled
up through _set_state and tore down engine init and reload_engine. That
turned PUT /api/config into a "Failed to fetch" for the user. The print
is now pipe-safe and api_server isolates the chained callback so a
future misbehaving listener cannot break the engine state machine.

Settings also now persists remote.email on login and shows a "Logged in
as <email>" indicator with a Log out button when an auth_token is
present, instead of leaving the email/password fields blank on reload.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/api_server.py

@@ -73,8 +73,15 @@ class APIServer:
         original_state_cb = self.controller.on_state_changed
 
         def on_state_changed(state: str, message: str):
+            # Isolate the upstream callback so a failure there (e.g. a
+            # broken stdout pipe in main_headless) cannot propagate into
+            # _set_state and tear down engine init / reload_engine /
+            # apply_settings request handling.
             if original_state_cb:
-                original_state_cb(state, message)
+                try:
+                    original_state_cb(state, message)
+                except Exception:
+                    pass
             self._broadcast_control({"type": "state_changed", "state": state, "message": message})
 
         self.controller.on_state_changed = on_state_changed
@@ -273,6 +280,7 @@ class APIServer:
                     data = resp.json()
                     ctrl.config.set('remote.auth_token', data.get('token', ''))
                     ctrl.config.set('remote.server_url', req.server_url)
+                    ctrl.config.set('remote.email', req.email)
                     return {"status": "ok", "token": data.get('token', '')}
                 else:
                     raise HTTPException(status_code=resp.status_code, detail=resp.text)