fix(ambient-mode): fix ambient-mode overlapping other elements

[h-banii]

Fixes #2520

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before

after

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More screenshots/explanation inside these spoiler tags:

audio-video toggle

.song-button.ytmusic-av-toggle, .video-button.ytmusic-av-toggle {
  z-index: 1;
  background-color: transparent;
}

The z-index brings it above the canvas and the background-color does this:

	default (#212121)	background-color: transparent
ambient mode
no ambient mode

side panel

#side-panel.side-panel.ytmusic-player-page {
  z-index: 0;
}

before	after

OBS.: The other tabs, lyrics and related, are also fixed by this.

toggle-disabled

the av toggle has a "toggle-disabled" attribute which makes it look faded/disabled, as the name suggests, so I removed it.

WITH toggle-disabled	NO toggle-disabled

I don't know how this attribute is actually used by youtube in the official web version (I don't have premium to test).