Improve support for expanding navbar channel description on touchscreens.

A few notes on things we could do better following #32702 at mobile sizes:

• https://github.com/zulip/zulip/pull/32702/commits/e133825502a517070eba840b5a968c01d4b04d39#r1908043677: Make it easier to trigger (I'm not sure if the current state is OK).
• Use the full width of the screen at mobile sizes, rather than just the portion that had been allocated to a slice of the channel description. Possibly it makes sense on mobile sizes to actually just have a button to display the channel description, rather than a compressed panel? I'm not sure, this needs some design.

The current state may not look good: