msglist: There should be 36px of scrollable whitespace at bottom (emphasizing that end-of-feed is reached)

[chrisbobbe]

As suggested by Vlad; see discussion: https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/flutter.3A.20Mark-as-read/near/1680603

[chrisbobbe]

Fixing this should mitigate an issue where tapping the mark-as-read button can surprisingly take you to a different message list, instead of marking messages as read: #327 (comment)

We should still fix the root cause of that (see link), but this should help make it happen less frequently, by getting the button out of the area with the buggy tap response when the message list is fully scrolled to the bottom.