Preserve stereo and fix mono/one-ear audio.
Makes sure YouTube videos always sound right in both ears without distorting stereo. It does this by checking the audio channels: if both left and right have sound, it leaves them untouched (true stereo); if one side is silent or the video is mono, it duplicates the active audio into both ears so it plays evenly. It uses very lightweight checks every few seconds, works automatically when YouTube loads a new video (since the site is single-page), and is designed to be efficient, cross-browser, and memory-safe.
Stereo untouched: no distortion, no unnecessary processing.
One-ear fixed: silent channel filled with audio.
CPU/RAM friendly: tiny FFT + infrequent checks.
Cross-browser: works in Chrome, Firefox, Safari.
