feat(sidebar): Desktop sidebars are now resizable

[gadenbuie]

This PR adds a resize handle into the sidebar element of layout_sidebar() to make all desktop (or wide-screen) sidebars resizable.

This is a Typescript and Sass-only change and does not require any R or Python updates.

Also, the initial implementation was written by Claude Sonnet 4 but I've reviewed all changes and made a few changes for code style and consistency and to improve the accessibility of the component.

Kapture.2025-06-11.at.13.39.26.mp4

[gadenbuie]

On second thought, non-collapsible desktop sidebars should be resizable, too. That starts to beg the question of adding an option, like resizable, but I think it's complicated enough that we should just have desktop sidebars always be resizable.

[cpsievert]

Nice! I'd like to have one more closer look before merging, but consider this basically approved. Let me know when you're ready to merge, and I'll have one more closer look

[gadenbuie]

Let me know when you're ready to merge, and I'll have one more closer look

I think you can go ahead with that look, the comments so far will be easy to address and don't involve any large changes

[cpsievert]

Great work, excited for this to land! 👏

[gadenbuie]

Thanks for the comments and review @cpsievert! Everything was easy to resolve, so I'm going to merge this.

Note: We have some flip-flopping happening with snapshots on Mac R 4.5.1 that I can't fully explain (hoping the issue will go away eventually, or we can fix as needed before next release).