Add hide watched videos setting

[Luzijano]

Summary

• implements the existing hide_watched_videos setting so watched YouTube recommendations/search/list items are hidden
• supports YouTube's watched overlay, resume-progress overlay, and ImprovedTube's stored watched video IDs
• adds a configurable watched-progress threshold slider (defaults to 90%) and re-applies hiding on navigation/dynamic page updates

Fixes #4128

Tests

• node -e "JSON.parse(require('fs').readFileSync('_locales/en/messages.json','utf8')); console.log('json ok')"
• npm test -- --runInBand tests/unit/hide-watched-videos.test.js
• npm run lint

[nkinnan]

I will test this PR tomorrow. Forgot to mention that (based on the behavior of other plugins) this may require different code paths for channel page versus player page. It should not affect playlists. I'll test all of this and let you know if I find any problems. Thanks for implementing this!