Fix agent thread sidebar covering window buttons

[OmChillure]

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:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Video

Screencast.from.2026-04-24.14-27-51.webm

Release Notes:

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

[danilo-leal]

Thank you so much!