fix: keep transcript visible when hiding sidebar

[nhatlong90]

Closes #1530. This keeps the transcript engagement panel visible when both Hide sidebar and Transcript are enabled, while still hiding related videos and other sidebar content. Added a unit test covering the CSS selector.

[ImprovedTube2]

This will disable most of 'hide sidebar'.
The two features can appear closer to each other (order of toggles)
and disable each other visibly (as done for other features, including the first two switches in at 'player')
(More precisely, then related videos: hidden and hide playlist, etc. should all activate. )

[ImprovedTube2]

Welcome to the team @nhatlong90