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>

backend/config.py

@@ -100,8 +100,7 @@ class Config:
             errors.append("SSH_PRIVATE_KEY is required")
         if not self.uptime_kuma_url:
             errors.append("UPTIME_KUMA_URL is required")
-        if not self.uptime_kuma_api_key:
-            errors.append("UPTIME_KUMA_API_KEY is required")
+        # UPTIME_KUMA_API_KEY is optional - can login via web UI
         if not self.claude_api_key:
             errors.append("CLAUDE_API_KEY is required")
         return errors