Always show scrollbars in tab panels when content overflows

[fluiddot]

Related to #225 (comment).

Proposed Changes

• Override tab panel styles to customize the scrollbar. This is the same approach we used for the sidebar but using different colors.

show-scrollbars.mp4

Testing Instructions

• Open the app.
• Resize the window to the minimum size.
• Navigate to the Settings tab.
• Observe the content overflows.
• Observe the scrollbar is always displayed.
• Observe the content can be scrolled.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[fluiddot]

@matt-west For the scrollbar color, I've used the same one the macOS was showing. For the hover state, I simply made the color 20% darker. Let me know if you'd like to adjust the colors before merging. Thanks 🙇 !

[matt-west]

Looks good. Thanks @fluiddot!

[wojtekn]

Thanks @fluiddot, the code change looks clear, and it works as expected. Now it's clearer for a user that the Settings panel can be scrolled down.