Fix datashader elements invisible with non-square aspect ratios

[timtreis]

Summary

• Fixes Unintuitive almost white color when plotting with datashader #445: datashader points (and other elements) appear invisible when the data has a non-square aspect ratio (e.g. after applying Scale([1, 5]))
• Root cause: _get_extent_and_range_for_datashader_canvas used np.min to compute the scaling factor, which made the canvas up to ~9× larger than the display on the longer axis. All pixel-based operations (spread, line_width) were then downscaled to sub-pixel size during display
• Fix: Changed np.min → np.max, ensuring the canvas never exceeds the display resolution on either axis

Details

With np.min, the canvas matched display pixels on the shorter axis but was oversized on the longer one:

Data 500×2500, Figure 640×480 → Canvas 640×3200 (8.7× oversized on y)
spread(px=1) → 3×3 canvas px → 3.0×0.4 display px → invisible

With np.max, the canvas is capped to display resolution:

Data 500×2500, Figure 640×480 → Canvas 100×500 (≤1× on both axes)  
spread(px=1) → 3×3 canvas px → 3×3 display px → visible

Verified no visual quality degradation for shapes, outlines, labels, images, or dense point clouds — the extra canvas pixels produced by np.min were always downscaled away during display and contributed nothing visible.

Test plan

• Added test_datashader_points_visible_with_nonuniform_scale regression test
• Visually verified shapes, outlines, labels, and images are unaffected
• Verified dense point clouds (50k pts) render identically
• Pre-existing image comparison failures are unrelated (also fail on main)

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.00%. Comparing base (6be1b41) to head (49bcda0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #544   +/-   ##
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  Coverage   84.00%   84.00%           
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  Files           9        9           
  Lines        2601     2601           
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  Hits         2185     2185           
  Misses        416      416           

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/utils.py	79.30% <100.00%> (ø)

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