UI updates and appearance customization #2

We want a Zed-inspired appearance first, then a way for users to reproduce and customize it without maintaining a fork.

Deliver this in two sequential sub-issues:

1. Implement the Zed-inspired design directly in this fork. Establish the actual appearance before designing configuration controls.
2. Audit those changes and make the required adjustments reusable through T3 Code's appearance features. The audit and implementation belong in one issue.

The second sub-issue depends on the first. Together they replace the separate Zed-styling and broad customization proposals.

Done when the new design is verified and users can reproduce it, customize it, share it, and restore defaults without editing source code. This parent tracks future work; creating these issues does not start implementation.