fix: defer uninstall to next restart when DLL is locked on Windows

[pabloinigoblasco]

Summary

• ExtensionManager: new signal uninstallPendingRestart, new public method applyPendingUninstalls(), private helper schedulePendingUninstall()
• MarketplaceWindow: subscribes to uninstallPendingRestart and shows the appropriate "restart required" feedback to the user

Problem

On Windows, uninstalling a plugin while it is loaded by the host process causes QDir::removeRecursively() to fail silently: the extension was not removed from disk, but the error was reported and the operation stopped there, leaving the user with no actionable path forward.

Solution

Align the uninstall flow with the existing install staging pattern. When removeRecursively() fails on Windows, uninstall() now:

1. Writes a .pj_pending_uninstall marker inside the extension directory
2. Removes the extension from the in-memory registry immediately (so it no longer appears as installed)
3. Emits the new uninstallPendingRestart(id) signal instead of an error

At the next application startup, applyPendingUninstalls() scans extensions_dir for directories carrying the marker and deletes them. If the directory still cannot be removed (unusual), it is silently skipped and retried on the following startup.

Test plan

• Run ./build.sh --debug && ./test.sh — all tests pass
• Verify new tests: ApplyPendingUninstallsRemovesMarkedDirectory, ApplyPendingUninstallsIgnoresUnmarkedDirectory, ApplyPendingUninstallsIsNoOpForEmptyDirectory
• On Windows: install plugin, attempt uninstall while loaded, verify "Needs Restart" badge appears, restart app and verify directory is removed

[pabloinigoblasco]

Superseded by #22, which is a unified reconstruction of the Windows marketplace adaptation work (staging for install/update/uninstall when DLLs are locked).