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 75.58140% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.88%. Comparing base (69105e0) to head (a2c765d).
⚠️ Report is 68 commits behind head on main.

Files with missing lines	Patch %	Lines
src/napari_spatialdata/utils/_utils.py	5.55%	17 Missing ⚠️
src/napari_spatialdata/_viewer.py	92.30%	3 Missing ⚠️
src/napari_spatialdata/_sdata_widgets.py	96.55%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #393      +/-   ##
==========================================
+ Coverage   63.08%   70.88%   +7.79%     
==========================================
  Files          19       19              
  Lines        2636     3400     +764     
==========================================
+ Hits         1663     2410     +747     
- Misses        973      990      +17     

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[asarigun]

Thanks again for the thorough review @LucaMarconato — all 15 threads addressed in a2c765d.

Highlights of this update:

• Checkboxes renamed to "Discard z for ..." with inverted semantics and a "new layers only" note in the tooltips.
• Visibility of the 3D Settings is now gated on SdataWidget._sdatas_have_z_axis(...), so 2D-only users don't see them at all.
• The z-range slider, get_z_range, filter_layers_by_z_range, the TestZBinning class, and the superqt import were all removed; happy to revisit as a follow-up (separate plugin / upstream napari).
• Test classes collapsed via pytest.parametrize per your suggestion.
• _get_transform now has a docstring explicitly stating that z is included only when include_z is truthy and the element / transformation has a z axis.

Ready for another look when you have time.

[LucaMarconato]

Thanks for the implementing the suggested changes, ready to merge! 😊