feat: Groups menu overflow shadows

[holloway]

To resolve #7528 this adds a box-shadow to indicate that more scrolling is available. This is a common UX pattern, and it requires JavaScript.

Here's how it looks,

• Light mode:
• Dark mode:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.79%. Comparing base (187c2c5) to head (b00dfd3).
Report is 327 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7702      +/-   ##
==========================================
- Coverage   88.98%   88.79%   -0.20%     
==========================================
  Files         291      296       +5     
  Lines       40717    41319     +602     
==========================================
+ Hits        36233    36688     +455     
- Misses       4484     4631     +147     

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[rjsparks]

There are a few other places where we have UI that has a scrollbar that could use this treatment - hopefully it generalizes and will be easy to apply in those spots (example, the various dropdowns on the search page).