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/ghostty_dolphin.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Type=Service
+ServiceTypes=KonqPopupMenu/Plugin
+MimeType=inode/directory
+Actions=RunGhosttyDir
+
+[Desktop Action RunGhosttyDir]
+Name=Open Ghostty Here
+Icon=com.mitchellh.ghostty
+Exec=ghostty --working-directory=%F --gtk-single-instance=false
+