Improve settings UX and tests

[kiannidev]

Summary

• Enhance Privacy settings page with proper loading, error handling, and optimistic toggle for 'Help improve Eigent'.
• Improve General settings Network Proxy UX with helper copy, change detection, and restart flow.
• Add unit tests for Privacy and General settings pages to cover loading, validation, and save behavior.

[4pmtong]

Thanks @kiannidev for contribution! could @a7m-1st help review it?

[bytecii]

Can you give a screenshot for your UI changes?

[kiannidev]

Thanks for the review @bytecii . This PR primarily improves **settings behavior and reliability**, so visual UI changes are intentionally minimal.

What changed visually

• General → Network Proxy: added a small helper text line explaining proxy behavior.
• General → Network Proxy: improved button states:
  • Save is disabled until the value is changed.
  • After save, action changes to Restart to Apply.

What changed functionally (main scope)

• Privacy setting toggle now has stronger loading/save handling and rollback on save failure.
• Improved error handling for privacy/proxy fetch and save failures.
• Added broader unit test coverage for Privacy and General proxy flows.