Selective sync toggle (off -> on) leaves mobile project in persistent pending state

Description
After disabling and then re‑enabling “Enable selective sync for mobile app” in QGIS plugin settings, the mobile app keeps reporting pending / unsynchronized changes even after multiple sync attempts. No real edits were made; the state should clear. This appears distinct from previously reported issues (#2942, #3245, #3129) which cover other selective sync stale states.

Environment
Staging

Application (+ app version, build, operating system)

• Mobile app version: 2025.6.0

Steps to reproduce

1. Clone public project tc08_selective_sync into your workspace so it can be synchronized to the mobile app.
2. Download/open the project in the mobile app.
3. In QGIS: Project -> Properties -> Mergin Maps. Uncheck “Enable selective sync for mobile app”. Click OK and synchronize.
4. In the mobile app: synchronize the project.
5. Back in QGIS: Project -> Properties -> Mergin Maps. Re‑check “Enable selective sync for mobile app”. Click OK and synchronize.
6. In the mobile app: synchronize the project once.
7. Observe the project still shows pending / unsynchronized changes.
8. Synchronize again: warning persists (no actual local modifications).

Actual results
Mobile app continually shows pending changes / needs sync indicator after the off→on toggle sequence despite successful sync attempts and no modifications.

Expected results
After re‑enabling selective sync and performing one sync, the mobile project should be marked up‑to‑date unless real edits exist.

Screenshots or logs

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

Start by reproducing the off-to-on selective sync sequence with the public project tc08_selective_sync, using the listed QGIS and mobile-app steps. Compare the mobile project's sync state after the first and second sync attempts, and inspect the selective-sync handling involved in the mobile app. Done means the project shows no pending changes after re-enabling selective sync when no real edits exist.