Fix categorical color mapping across coordinate systems

[timtreis]

Summary

• Fixes ValueError: zip() argument 2 is shorter than argument 1 when plotting shapes colored by a categorical column across multiple coordinate systems with different category counts
• Calls remove_unused_categories() early in _generate_base_categorial_color_mapping and _extract_colors_from_table_uns so that categories and stored .uns colors stay aligned
• Uses category position in the full table (not the subset) for color lookup, ensuring consistent coloring across coordinate systems

Fixes #425

Test plan

• Verify plotting shapes with color="leiden" across multiple coordinate systems where category counts differ no longer crashes
• Verify colors remain consistent (same category gets same color) regardless of which coordinate system is being rendered
• Verify no regression when all categories are present in every coordinate system

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 42.85714% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.86%. Comparing base (a2bb56b) to head (ec09641).

Files with missing lines	Patch %	Lines
src/spatialdata_plot/pl/utils.py	42.85%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #547      +/-   ##
==========================================
- Coverage   84.00%   83.86%   -0.15%     
==========================================
  Files           9        9              
  Lines        2601     2609       +8     
==========================================
+ Hits         2185     2188       +3     
- Misses        416      421       +5     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/utils.py	79.08% <42.85%> (-0.23%)	⬇️

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