chore(dist): add Linux desktop integration files for installation

Desktop entry, AppStream metainfo, D-Bus service and systemd user unit
templates plus icons, adapted from ZIG/dist/linux and used by
`just install`.

dist/linux/app.desktop.in

@@ -0,0 +1,26 @@
+[Desktop Entry]
+Version=1.0
+Name=@NAME@
+Type=Application
+Comment=A terminal emulator
+TryExec=@GHOSTTY@
+Exec=@GHOSTTY@ --gtk-single-instance=true
+Icon=com.mitchellh.ghostty
+Categories=System;TerminalEmulator;
+Keywords=terminal;tty;pty;
+StartupNotify=true
+StartupWMClass=@APPID@
+Terminal=false
+Actions=new-window;
+X-GNOME-UsesNotifications=true
+X-TerminalArgExec=-e
+X-TerminalArgTitle=--title=
+X-TerminalArgAppId=--class=
+X-TerminalArgDir=--working-directory=
+X-TerminalArgHold=--wait-after-command
+DBusActivatable=true
+X-KDE-Shortcuts=Ctrl+Alt+T
+
+[Desktop Action new-window]
+Name=New Window
+Exec=@GHOSTTY@ --gtk-single-instance=true