Add web-based Uptime Kuma login with TOTP support

- Add login modal for username/password/TOTP authentication
- Persist token to file for session persistence
- Make UPTIME_KUMA_API_KEY optional (can login via web UI)
- Add /api/kuma/auth, /api/kuma/login, /api/kuma/logout endpoints
- Show login prompt when not authenticated

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/api/client.js

@@ -61,4 +61,10 @@ export const api = {
 
   // Uptime Kuma
   testKumaConnection: () => fetchApi('/kuma/test'),
+  getKumaAuthStatus: () => fetchApi('/kuma/auth'),
+  kumaLogin: (username, password, totp) => fetchApi('/kuma/login', {
+    method: 'POST',
+    body: JSON.stringify({ username, password, totp }),
+  }),
+  kumaLogout: () => fetchApi('/kuma/logout', { method: 'POST' }),
 };