Code Quality: TreeView sidebar cleanup, async folder enumeration, and…

[workbysaran]

Resolved / Related Issues
Cleanup and stability fixes for the experimental TreeView sidebar (introduced in branch ya/TreeViewConcept):

• Removes a closed-tab memory leak in the per-tab expansion-state dictionary.
• Moves folder enumeration off the UI thread so an unreachable network drive or unresponsive USB device no longer freezes the sidebar on startup or on expand.
• Several mechanical cleanups (dead code, redundant qualifiers, hardcoded English strings, converter file location) to align with the project's existing patterns.

Steps used to test these changes

1. Enable "TreeView sidebar (experimental)" in Settings >> Dev tools.
2. Open multiple tabs, expand different folders in each, switch between tabs and verify per-tab expansion state is preserved.
3. Close tabs that had expanded folders. Verify the closed tab's state is released (no slow leak across many tab open/close cycles).
4. Right-click sidebar items, verify Open / Open in new tab / Copy path / Open in File Explorer menu items work and are localized.
5. Expand a folder with many subdirectories (e.g. "C:\Windows\System32") and verify the UI stays responsive while children load.