Add linewidth support to legend inputs

[K-Mirembe-Mercy]

Summary

Adds support for "linewidth" in legend inputs.

Changes

• Passes "linewidth" from kwargs into "_LegendInputs"
• Allows users to control legend frame linewidth

Notes

• Minimal change, no breaking behavior

[cvanelteren]

This PR is not complete yet. Have a look at our previous commits on how they are structured and our CONTRIBUTING docs or https://ultraplot.readthedocs.io/en/latest/contributing.html

A PR will have the following elements

• Does it fix an issue?
• Does it contain test for the patched code?
• Does it need documentation?
• Does it need examples in the gallery?

The PR here does not follow the same pattern for the other line widths that were introduced for semantic legends.

[cvanelteren]

Also do we need to close #654 ?

[K-Mirembe-Mercy]

Thanks for the feedback!

I’ll go through the contributing guidelines and review how linewidth is implemented in other parts of the codebase so I can align this PR with the existing pattern.

I’ll also work on adding tests, documentation, and examples as needed.

Regarding #654, I’ll verify whether this PR fully addresses it before deciding to close it.

Appreciate the guidance — still learning 🙂