Filter unused categories from clonotype network legend

[LiudengZhang]

Summary

• The clonotype network plot legend now only shows categories that are actually present in the plotted data
• Previously, all categories from the full dataset were listed, making it hard to identify colors when only a subset of categories appeared in the network
• Collects which category colors are used in the pie chart rendering, then filters the legend entries accordingly

Closes #321

Test plan

• Verify legend only shows categories present in the plotted network subset
• Legend still works correctly when all categories are present
• No visual regression for non-categorical coloring (continuous variables, uniform color)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.31%. Comparing base (0f68a0a) to head (58020ff).

Files with missing lines	Patch %	Lines
src/scirpy/pl/_clonotypes.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #680      +/-   ##
==========================================
- Coverage   19.33%   19.31%   -0.03%     
==========================================
  Files          51       51              
  Lines        4628     4633       +5     
==========================================
  Hits          895      895              
- Misses       3733     3738       +5     

Files with missing lines	Coverage Δ
src/scirpy/pl/_clonotypes.py	10.87% <0.00%> (-0.20%)	⬇️

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