feat: add Volume Boost player button (addresses #4146)

[flujopy2025-beep]

Summary

Adds a Volume Boost button to the player controls that amplifies audio beyond 100% using the Web Audio API (GainNode).

Changes

• New player button: click cycles through 1x -> 2x -> 3x -> 1x amplification
• Button opacity indicates active state (dim = 1x, bright = boosted)
• Gain resets to 1x when feature is disabled in settings
• Toggle available under Player section in extension menu
• Uses standard Web Audio API: AudioContext + GainNode (no dependencies)

How it works

1. User enables 'Volume Boost' in Player settings
2. A speaker icon button appears in the player controls
3. Clicking it cycles the audio gain: 1x (normal) -> 2x -> 3x -> back to 1x
4. Useful for quiet videos, podcasts, or when device volume is maxed out

Testing

• Tested on YouTube watch pages with various volume levels
• AudioContext is created lazily on first click (avoids autoplay policy issues)
• Works alongside existing volume slider (boost multiplies the slider value)

References

Addresses the 'Volume Boost' item from issue #4146 (Player buttons to be added).
Also related to #1382 and the Volume Boost section in the issue's specification.