chore: Fix theme refresh, popup layering, and title bar styling (#2701)

## Summary

- add a subtle theme-derived gradient to the default title bar
background
- synchronize custom theme changes into `gpui-base` so menus, resize
handles, and input focus rings refresh correctly
- give popup surfaces a shared priority above dialogs, including nested
menu and picker surfaces
- include the accompanying bundled theme palette updates and remove the
Matrix theme

## Test Plan

- `cargo fmt --check`
- `cargo test`

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Co-authored-by: Codex <codex@openai.com>