Sync starts despite invalid filename warning; ends with “Unsupported file detected” and changes not synchronized

Description
When a project contains a file whose name includes invalid/unsupported characters, the Project status dialog shows a warning, but the user is still allowed to start synchronization. The sync runs and finishes with an error “Unsupported file detected.” The changes (including valid ones) are not synchronized. This leads to wasted time and unclear messaging (two different phases: warning then late failure).

Steps to reproduce

1. Open a Mergin Maps project locally in QGIS.
2. Place or create a file in the project directory whose name contains characters invalid or unsupported by the plugin/server sync (example: survey:raw?.gpkg or includes characters like :*?<>| on Windows).
3. Open the Mergin Maps Project status dialog.
4. Observe a warning about the problematic/unsupported file (naming or unsupported type).
5. Start sync anyway.
6. Wait for the sync process to complete.

Actual results

• Sync is allowed to start despite the warning.
• Process finishes with an error: “Unsupported file detected.”
• Project changes are not synchronized (including otherwise valid modifications).
• User receives late failure instead of an early block; error message does not explicitly list the offending file(s).

Expected results
Sync cannot start while invalid/unsupported filenames are present. A clear, early blocking error lists all offending files and instructs the user to rename/remove them. No network operations are started.

Screenshots or logs

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

No source files, tests, or entry points are named in the issue. Trace the Project status dialog's sync-start path and existing unsupported-file warning, then add coverage for blocking before network operations and listing all offending files; done means valid changes do not start syncing while invalid names remain.