This PR converts the collab panel list from a GPUI list into a uniform
list, which was a requirement for the indent guides to be displayed. I
have been wanting to add that to the channels list for a while, and
finally carved out some time. Ended up adding some more tweaks here and
there as overall improvements (e.g., using a lock icon for private
icons) and to make the indent guide fully work design-wise given we have
the chevron buttons rendering on top of them.

Here's how it looks:

<img width="600" alt="Screenshot 2026-07-21 at 10  08@2x"
src="https://github.com/user-attachments/assets/351b7e0f-aeb4-4c79-8177-7f1cdf635fbe"
/>

Release Notes:

- N/A

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Co-authored-by: MrSubidubi <finn@zed.dev>