Android: Sync shows no error when server returns 503 Service Temporarily Unavailable

Description
When syncing a project on Android while the server is temporarily unavailable (HTTP 503), the app provides no visible user-facing error (no toast/banner). The sync action appears to do nothing, leaving users uncertain about the failure and the server’s status.

Environment
Staging

Application (+ app version, build, operating system)

• App: Mergin Maps Mobile (Android)
• App version: 2025.6.0 (build 700311)
• Device/OS: Samsung Galaxy A53 5G (Android 15), Motorola G62 (Android 13)
• Network: Device online (Wi‑Fi/Cellular); server endpoints return HTTP 503 Service Temporarily Unavailable

Steps to reproduce

1. Ensure the Android device has internet access.
2. Make the Mergin Maps server return HTTP 503 for sync-related API calls (e.g., maintenance mode or a test/proxy that responds 503).
3. In the app, open a project and tap Sync.
4. Observe the UI feedback.

Actual results

• No visible error message is displayed (no toast/banner/dialog).
• The sync action appears idle or silently fails with no feedback to the user.

Expected results

• A clear, immediate user-facing message indicating “Server temporarily unavailable (503)” or a generic “Server is temporarily unavailable. Please try again later.”
• Action should fail fast with a visible message and an option to Retry when appropriate.
• Consistent handling for 5xx responses, distinct from offline/airplane-mode behavior.

Screenshots or logs

Conclusion
Created by GitHub Copilot and supervised by a human test analyst.

Start at the Android project sync entry point and reproduce the failure against a server returning HTTP 503. Trace how the sync response is surfaced to the UI; done means a visible temporary-server-unavailable message, an appropriate retry option, and behavior distinct from offline mode.