- Backend: new endpoint GET /auth/status returns {configured: bool}
- Webclient auth store: add authConfigured ref + fetchStatus()
- LoginScreen.vue: centered card with logo, title, and login button
- App.vue: show LoginScreen overlay when auth is configured but
  user is not authenticated (z-index 9999, blocks all UI)
- App.vue onMounted: fetch auth status before trying to resolve user

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>