fix: import scanpy default palettes across scanpy versions (1.13 compat)

[timtreis]

Problem

The py3.14-pre CI env installs pre-release scanpy 1.13.0a1, where import spatialdata_plot crashes at collection:

src/spatialdata_plot/pl/_palette.py:23: in <module>
    from scanpy.plotting.palettes import default_20, default_28, default_102
E   ModuleNotFoundError: No module named 'scanpy.plotting.palettes'

Every test in that env errors at conftest.py import.

Root cause

scanpy 1.13 relocated the default categorical palettes from scanpy.plotting.palettes to scanpy.plotting.legacy.palettes (no surviving sc.pl.palettes alias). spatialdata-plot imported the old path in three modules (_palette.py, _color.py, utils.py).

Fix

Add a single _scanpy_palettes shim that imports from the new path and falls back to the old one, and point all three modules at it:

try:  # scanpy >= 1.13
    from scanpy.plotting.legacy.palettes import default_20, default_28, default_102
except ImportError:  # scanpy < 1.13
    from scanpy.plotting.palettes import default_20, default_28, default_102

The palette values are frozen — default_20/28/102 hash identically across scanpy 1.11.4 and 1.13.0a1 — so behavior is unchanged; this only makes the import resilient to the relocation.

Verification

• Shim resolves on stable 1.11.4 (fallback path) and pre-release 1.13.0a1 (new path); both yield 20/28/102 colors.
• import spatialdata_plot succeeds; test_palette.py (41) green locally.

Notes

py3.14-pre is continue-on-error (non-gating), so this is a forward-looking fix for scanpy 1.13 readiness rather than a merge blocker.