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/main_headless.py

@@ -75,10 +75,16 @@ def main():
         # Create controller and initialize
         controller = AppController(config=config)
 
-        # Wire a state callback that prints the ready event
+        # Wire a state callback that prints state events for the parent
+        # process to read. Stdout writes can fail with EINVAL on Windows
+        # when the parent stops reading the sidecar pipe; swallow those
+        # so the engine state machine isn't taken down by a logging path.
         def on_state_changed(state, message):
             event = {"event": "state", "state": state, "message": message}
-            print(json.dumps(event), flush=True)
+            try:
+                print(json.dumps(event), flush=True)
+            except (OSError, ValueError):
+                pass
 
         controller.on_state_changed = on_state_changed