Added feature to mute thumbnil previews

[pswitchy]

📋 Problem

YouTube natively mutes thumbnail previews only on initial page refresh. When hovering over a different video, the preview can start playing with audio—which is intrusive and annoying for users browsing their feed.

💡 Solution

Enhanced the existing muteThumbnailPreviews feature to forcefully mute every new thumbnail preview on hover, not just on initial page load.

---

🛠 Changes

js&css/extension/www.youtube.com/general/general.js

• Added forceMute(): Attaches volumechange and play event listeners to each preview <video> element to immediately re-mute if YouTube's code tries to unmute it.
• Added isPreviewVideo(): Helper for cleaner preview container detection.
• Optimization: Uses _itMuteEnforced flag to ensure listeners are only attached once per video element.
• MutationObserver: Now watches for src attribute changes to catch when YouTube reuses the same <video> element for a different hover preview.

js&css/web-accessible/www.youtube.com/playlist-cleaner.js (New)

• Added missing stub file referenced in manifest.json web_accessible_resources—its absence was preventing the extension from loading.

---

⚙️ How It Works

1. Initial Load: All existing preview videos are force-muted.
2. Observation: A MutationObserver watches for new preview <video> elements and src attribute changes.
3. Enforcement: Each preview video gets volumechange + play listeners that re-mute instantly if audio is restored.
4. Scoped Impact: Only preview containers are targeted (#inline-preview-player, ytd-video-preview, etc.)—the main player is unaffected.

---

✅ Testing

• All 25 existing unit tests pass
• Manually verified it's working