Use consistent header across markbook assignments/tests

[jacbn]

Moves the (assignment) markbook header into its own component and reuses this across both assignments and tests.

Main intention is for Ada's "Show as percentages" to appear on tests; also ensures future consistency.

Also applies minor improvements to the Groups page.

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 41.34%. Comparing base (955b53b) to head (4e5e350).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
.../components/pages/AssignmentProgressIndividual.tsx	18.18%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1751      +/-   ##
==========================================
- Coverage   41.34%   41.34%   -0.01%     
==========================================
  Files         533      533              
  Lines       23430    23434       +4     
  Branches     7757     7778      +21     
==========================================
+ Hits         9688     9689       +1     
- Misses      13108    13111       +3     
  Partials      634      634              

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