Windows: sync fails with [WinError 32] when remote user deletes a GeoPackage that is open in QGIS

Description
When a project is open in QGIS on Windows and another user (e.g. on macOS) deletes a GeoPackage file from the project, saves and syncs, the Windows client fails to apply the pull (delete) action and shows a client error instead. The GeoPackage file is locked by QGIS which prevents the plugin from deleting it during sync.

Error: Client error: Detail: Failed to apply pull actions: [WinError 32] The process cannot access the file because it is being used by another process

This issue was previously reported in #334 but was closed as duplicate. It appears to still be reproducible with current versions.

Environment
[x] PROD

Application (+ app version, build, operating system)

• QGIS: 3.40.15-Bratislava
• Mergin Maps Plugin version: TBD
• macOS: Tahoe 26.3 (syncing machine)
• Windows 11 (affected machine)

Steps to reproduce

1. macOS: create a new project in Mergin Maps plugin with a GeoPackage layer
2. Windows: download and open the project in Mergin Maps QGIS plugin
3. macOS: delete the .gpkg file from the project, save and sync
4. Windows: sync the project in the Mergin Maps QGIS plugin → Client error appears

Actual results
Sync fails with [WinError 32] The process cannot access the file because it is being used by another process

Expected results
The plugin should handle the locked file gracefully — either close/unload the layer before deleting it during pull, or show a clear message asking the user to close the layer first.

Screenshots or logs

Conclusion
Created by Claude and supervised by a human test analyst.

Start by reproducing the Windows sync scenario from the issue and trace the plugin's pull-action handling for deleted GeoPackage files while the layer remains open in QGIS. Done means the locked-file case no longer produces an unhandled client error: the layer is unloaded before deletion or the user receives a clear instruction to close it.