## Summary

Fixes an oversight in the agent threads archive sidebar's header: on
Linux and Windows it only accounted for macOS traffic lights, so the
custom window controls (minimize/maximize/close) rendered on top of the
sidebar. The main sidebar
(`crates/sidebar/src/sidebar.rs::render_sidebar_header`) already handles
this correctly — this PR mirrors that pattern in
`ThreadsArchiveView::render_header`.

### What changed
- `crates/agent_ui/src/threads_archive_view.rs`
- `render_header` now computes `traffic_lights`, `left_window_controls`,
and `right_window_controls` (matching `sidebar.rs`) and reserves space /
renders controls accordingly when not fullscreen.
- Added `render_left_window_controls` / `render_right_window_controls`
helpers that delegate to `platform_title_bar`.
  - Imported `workspace::CloseWindow`.
- `crates/agent_ui/Cargo.toml`
  - Added `platform_title_bar` dependency.

### Behavior
- macOS (sidebar on left): unchanged — traffic-light padding + divider.
- Linux/Windows, sidebar on left: renders left-side window controls
inside the header.
- Linux/Windows, sidebar on right: renders right-side window controls
inside the header.
- Fullscreen: no controls rendered, same as before.

#### Closes #54596

### Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

### Video 

[Screencast from 2026-04-24
14-27-51.webm](https://github.com/user-attachments/assets/47009458-2acb-4b65-bff6-d25302a0342e)

Release Notes:

- Agent: Fixed the threads sidebar overlapping the window control
buttons on Linux and Windows.

Co-authored-by: Danilo Leal <67129314+danilo-leal@users.noreply.github.com>