On Linux, `titleBarStyle: 'hidden'` removes the title bar entirely,
leaving no window controls (close/minimize/maximize). macOS has traffic
lights and Windows has a custom title bar component, but Linux gets
nothing. Fix this by omitting the hidden title bar style on Linux,
which gives native window decorations from the desktop environment
(GNOME, KDE, Hyprland, etc.).
Also adds Linux build support:
- electron-builder `linux` config targeting pacman (.pkg.tar.zst)
- `dist:linux` script for standalone Linux builds
- `release-linux` CI job on ubuntu-latest
- `dist` script updated to include --linux
50d5b922b6