feat(plotting): shared axis labels for faceted plots

[FBumann]

Summary

Plotly Express repeats the x-axis title under every facet column and the y-axis title beside every facet row. Plotly has no flag to fix this on an existing figure — the only built-in (make_subplots(x_title=..., y_title=...)) works at figure creation only and is not reachable through px (long-standing community request).

This PR makes faceted plots render one shared, centered label per axis instead:

• New public helper share_axis_labels(fig): collapses repeated, identical axis titles into a single label per axis, usable on any figure (raw px, overlay output, etc.)
• New shared_axis_labels kwarg on all plotting/accessor methods, on by default; pass shared_axis_labels=False for plotly's stock behavior

Design

• The added labels use annotation specs byte-identical to plotly's own make_subplots(x_title=, y_title=) implementation — a sync test compares against real make_subplots output, so if plotly ever changes its styling we'll know
• Titles are only collapsed when repeated and identical:
  • non-faceted figures → unchanged
  • subplots() combinations with differing titles → unchanged
  • secondary-y (overlaying) axes → never collapsed
• Applied after simplify_facet_titles, so facet title post-processing and shared labels compose

Visual

Before	After
"Air Temperature [K]" ×2 rows, "time" ×2 columns	one of each, centered

Test plan

• 11 new tests: helper behavior per facet direction, idempotency, no-facet no-op, secondary-y safety, differing-title safety, kwarg default/opt-out, imshow facets, and the make_subplots sync test
• Existing imshow facet tests updated (annotation sets now include shared labels)
• ruff, ruff format, mypy clean; 173 tests pass
• Visually verified renders (facet grid, facet_col-only) via kaleido

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 16fbbe45-b25e-49a2-9361-5882dafdab97

📥 Commits

Reviewing files that changed from the base of the PR and between 10cfda0 and e720f47.

📒 Files selected for processing (6)

• tests/test_accessor.py
• tests/test_figures.py
• xarray_plotly/__init__.py
• xarray_plotly/accessor.py
• xarray_plotly/figures.py
• xarray_plotly/plotting.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/shared-facet-axis-labels

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