fix(manager): remove stale plugins with missing directories

[anhkhoakz]

Summary

Even the plugin directory missing, Yaak plugin manager still count it and try to exec it. Causing this error Failed to load plugin "yaak-plugin": IO error: No such file or directory (os error 2)

Submission

• This PR is a bug fix or small-scope improvement.
• If this PR is not a bug fix or small-scope improvement, I linked an approved feedback item below.
• I have read and followed CONTRIBUTING.md.
• I tested this change locally.
• I added or updated tests when reasonable.

Approved feedback item (required if not a bug fix or small-scope improvement):

Related

[gschier]

Thanks for the fix here!

I changed the shape a bit because I didn't want Yaak to automatically uninstall plugins when their directory is missing (path could be temporarily unavailable, such as from an unmounted disk).

Instead, I updated the plugin listing flow so missing plugins still appear in the Installed Plugins tab, where the user can uninstall them manually if they want.