Fix slow minimize and polish UI

- Replace spawn-tray-shell with in-process tray icon on Linux and Windows,
  making minimize-to-tray instant instead of waiting for a new process
- Add TrayVisibilityCommand::Show and background control thread for Windows
- Increase title bar height, button sizes, and corner radii for a more
  polished look
- Deepen panel backgrounds for better visual hierarchy
- Add section title underlines, accent color bar on status card
- Wrap log area in its own dark frame
- Bump global font sizes slightly for readability
- Add window shadow to theme

Author: fjh1997