Add a new template parameter to ridgeplot()

[tpvasconcelos]

Closes #238

Summary

This PR adds a new template parameter to ridgeplot(), allowing users to theme their plots with Plotly figure templates:

fig = ridgeplot(samples=..., template="plotly_dark")

• The parameter accepts the same representations as Plotly Express: the name of a registered template (including "+"-merged names, e.g. "plotly_white+ggplot2"), a go.layout.Template object, or a dictionary with a template's properties. Validation/coercion is delegated to Plotly's own BaseTemplateValidator, so error messages match Plotly's.
• Consistent with the existing behaviour for the default template, the default colorscale is now inferred from the specified template (i.e., from template.layout.colorscale.sequential), unless the user explicitly sets a custom colorscale, which still takes precedence.
• When template is not specified (default), the behaviour is completely unchanged: the figure is rendered with Plotly's current default template (plotly.io.templates.default) and the default colorscale is inferred from it (the existing e2e artifacts remain byte-identical).

Changes

• src/ridgeplot/_types.py: new PlotlyTemplate type alias
• src/ridgeplot/_template.py: new module with validate_coerce_template()
• src/ridgeplot/_color/colorscale.py: infer_default_colorscale() and validate_coerce_colorscale() now accept an optional resolved template
• src/ridgeplot/_figure_factory.py: plumb the new template argument through create_ridgeplot() and apply it to the figure's layout
• src/ridgeplot/_ridgeplot.py: new public template parameter (+ docstring)
• New unit tests covering all accepted representations, error cases, and the interaction with colorscale (100% diff coverage)

Test plan

• pytest tests/unit --doctest-modules src (251 passed)
• pytest tests/e2e/test_examples.py::test_json_regressions (artifacts unchanged)
• pyright (0 errors)
• pre-commit hooks (incl. ruff format/lint)
• Sphinx docs build with no new warnings (cross-references to Plotly's API docs resolve correctly)

---

📚 Documentation preview 📚: https://ridgeplot--389.org.readthedocs.build/en/389/

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (658306c) to head (5bdfdab).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #389   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files           18        19    +1     
  Lines          657       674   +17     
  Branches        85        88    +3     
=========================================
+ Hits           657       674   +17     

Flag	Coverage Δ
combined-src	100.00% <100.00%> (ø)

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