feat: add backgroundSidebar theme property for independent configuration of the sidebar's background colour (resolves #5286)

[ariane-emory]

This PR adds a new, optional backgroundSidebar theme property that:

• Allows independent theming of sidebar background separate from chat message panels
• If not present in the theme, falls back to backgroundPanel for backward compatibility
• Updates core theme system with proper type definitions and resolution logic
• Updates TUI sidebar component to use new theme property
• Does NOT require modification built-in themes or users' custom themes - they use fallback logic automatically

This enables users to theme sidebar background independently while maintaining full backward compatibility with all existing themes (both stock and custom).

Resolves #5286.

[jesseleite]

Yesss 🎉

Also, thinking of the existing naming conventions, do you think it should it be backgroundSidebar maybe?

[ariane-emory]

Sure, I can change the name if you'd like. There's also a test GH action failing presently, as you can see on this page. Gimme little longer get that done, OC somehow trounced part of my usual dev env and putting it back together might take me another hour or two.

[jesseleite]

Sure, I can change the name if you'd like.

That's probably more of a Dax/Adam question though! ❤️

[ariane-emory]

Corrupted by a bad merge, will fix.

[ariane-emory]

Uncorrupted.