The Windows title bar overlay (minimize/maximize/close buttons) was
hardcoded to dark colors and did not update when switching themes.
- Add titleBarTheme helper that maps AppearanceTheme to overlay and
background colors, using nativeTheme for the system preference
- Call applyTitleBarTheme from the setTheme IPC handler so the overlay
updates immediately on theme change
- Apply persisted theme on startup so the overlay matches from launch
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>