Fix watcher idle resume and cancellation

[marcmy]

Summary

• keep idle notifications active while a watcher run is in progress
• serialize watcher runs so repeated idle events cannot overlap
• use one cancellation path for idle and manual runs
• let the background worker dispose its own resources after it has stopped
• always reset folder state and process priority in cleanup

Validation

• dotnet restore CompactGUI/CompactGUI.vbproj succeeded on windows-latest
• dotnet build CompactGUI/CompactGUI.vbproj --configuration Release --no-restore succeeded

Runtime test plan

1. Let an idle-only watcher run begin.
2. Use the mouse or keyboard so the run pauses.
3. Become idle again and confirm it resumes.
4. Cancel during analysis and during compression.
5. Confirm the UI returns to idle and another watcher run can start.