Add full-screen login overlay for unauthenticated users ...

- 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>

Multi-user auth via gnexus-auth OAuth + hybrid role/permission model ...

- Integrate gnexus-auth-client-py (GAuthClient) for OAuth flow, token refresh,
  and webhook parsing
- Add navi/auth/ package: User model, Fernet encryptor, client singleton,
  deps (get_current_user, require_admin, require_permission)
- New tables: navi_users, user_auth_sessions (auto-created on startup)
- Session/memory isolation by user_id with legacy NULL support
- Cookie-based auth proxy: /auth/login, /callback, /logout, /me
- Webhook receiver /webhooks/gnexus-auth handling user events, global logout,
  session revocation, role/permission changes
- Admin endpoints (/admin/*) gated by role + permissions
- Webclient auth store with isAdmin/hasPermission guards
- Admin-only profile filtering in /agents/profiles
- 200/200 tests passing

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