feat(player): add Audio-Only mode with static thumbnail overlay (#4077)

[Monu01123]

Summary

Resolves #4077

Adds an Audio-Only Mode to Improve YouTube! that hides the video stream and displays a clean static album art / thumbnail overlay while listening. This dramatically reduces CPU/GPU frame decoding overhead and saves >99% bandwidth by switching to the lowest video bitrate (tiny / 144p).

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Features Implemented

1. Player Controls Toggle Button (#it-audio-only-button)

   • Added a quick-toggle headphones button directly to the YouTube player controls bar alongside the repeat and screenshot buttons.
   • Allows users to switch seamlessly between video and audio-only modes without refreshing the page.

2. Static Album Art Overlay (#it-audio-only-overlay)

   • Replaces the video rendering area with a high-resolution thumbnail / album art container layered over a smooth, blurred background.
   • Displays track title, channel/artist name, and an 🎧 Audio-Only Mode badge.
   • Styled with pointer-events: none; so all standard YouTube player interactions (play/pause clicks, double-click for fullscreen, scrubber bar dragging) function naturally.

3. CPU, GPU & Bandwidth Optimization

   • CPU/GPU: Sets opacity: 0 !important; visibility: hidden !important; on the <video> element, signaling the browser rendering engine to skip video frame decoding and compositing during every vblank.
   • Bandwidth: Automatically forces the player quality to tiny (144p / ~30 kbps) when Audio-Only mode is enabled. Restores the user's previous quality preference when disabled.

4. Full Playlist Support

   • Hooked into YouTube SPA navigation events (yt-navigate-finish and yt-page-data-updated).
   • When listening to playlists or albums, track transitions automatically maintain the audio-only state and dynamically update the album art and track metadata.

5. Optional Auto-Switch for Music Videos

   • Implemented intelligent music video detection (ImprovedTube.isMusicVideo()) checking meta[itemprop=genre], microformat renderer schemas, and artist channel identifiers (VEVO / - Topic).
   • Added an optional menu toggle (Auto-switch for music category) to automatically switch to Audio-Only mode whenever a music video is played.

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UI Settings & Localization

• Added toggle switches under Player -> Settings and Player -> Buttons.
• Added English localization strings (audioOnlyMode, audioOnlyButton, autoSwitchForMusic) to _locales/en/messages.json.

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Verification & Testing

• ✔ Verified toggle button appearance and state persistence across page navigation.
• ✔ Verified network bandwidth drop when switching to tiny quality.
• ✔ Verified clean UI layout in standard, cinema, and fullscreen modes.