Make scroll bar for accounts in catch-up taller, give it more whitespace, and do not fade it out.

**Is your feature request related to a problem? Please describe.**
The scroll bar for accounts in the catch-up window is quite flat, crammed between other interactive elements and fades out.

**Describe the solution you'd like**
Make the scroll bar taller, give it more whitespace above and below, and do not fade it out.

**Describe alternatives you've considered**
The scroll bar is the only means to navigate the account avatars ribbon. Another option might be to optionally show a block of all avatars, with appropriate line breaks.

**Additional context**
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Start by locating the catch-up window and its account avatars ribbon in the JavaScript frontend. Inspect how the ribbon's scrollbar is sized, spaced, and faded, then verify the UI manually. Done means the scrollbar is taller, has whitespace above and below, remains visible, and still navigates the avatars.

Written by the indexing model from the issue text.