AuthGate component wraps the app layout:
- On load, checks if saved token is valid by calling a protected endpoint
- If auth not required (dev mode), passes through immediately
- If auth required and no valid token, shows a clean login screen
- Token input with password field, stored in localStorage on success
- Shows guidance: "Find your token in secrets.yaml under api.bearerToken"
- Handles connection errors gracefully (shows app anyway)
