Support 3D visualization for points and 2.5D shapes

[asarigun]

Closes #31.

Builds on #366 to complete 3D support for vector data:

• 2.5D circles rendered at correct z depth
• Affine transforms respect z axis on coordinate system switch
• Save round-trip no longer drops z from points/shapes
• UI toggles for 3D points and 2.5D shapes projection
• Z-range slider to filter by depth

[codecov]

Codecov Report

❌ Patch coverage is 85.54913% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.46%. Comparing base (69105e0) to head (8e8a83b).
⚠️ Report is 66 commits behind head on main.

Files with missing lines	Patch %	Lines
src/napari_spatialdata/utils/_utils.py	0.00%	15 Missing ⚠️
src/napari_spatialdata/_viewer.py	90.90%	9 Missing ⚠️
src/napari_spatialdata/_sdata_widgets.py	98.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #393      +/-   ##
==========================================
+ Coverage   63.08%   71.46%   +8.38%     
==========================================
  Files          19       19              
  Lines        2636     3491     +855     
==========================================
+ Hits         1663     2495     +832     
- Misses        973      996      +23     

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