fix: visualization of multiple attachments

[madbob]

The original issue occurs on mobile, when expanding the "more attachments" area. I've introduced an explicit scrollable area that permits to navigate them.

The most invasive modification here is the change of the background color, which was a linear gradient and now is a static color. A linear gradient is proportional, and the height of the transparent portion is unpredictable as may vary accordingly to the number of attachments. With many attachments, the transparent area growns and interact badly with the overlapped contents of the message.

I've also introduced some minor adjustments to bottom buttons ("View more attachments", "Show all on Files", "Download zip") alignment on mobile.

(Don't care broken attachment previews in my screencasts: I'm breaking stuff around 😛 )

Original state:

test-original.mp4

New state:

test-mine-changes.mp4

Fixes #10408

[ChristophWurst]

Tested with responsive mode of FF for Linux & works well

The only nitpick is consideration of using multiple of var(--default-grid-baseline) instead of magic numbers

[madbob]

Indeed, you are so right that I've just removed the existing clickable elements and have replaced them with proper NcButton for improved themability and accessibility. So there is no need at all to hard-wire spacing and margins for them.