Add nautilus preload service for faster file manager startup

xdg-open ~ (Super+E) cold-starts Nautilus every time, causing
noticeable delay. A GApplication service keeps Nautilus warm in the
background so subsequent opens are near-instant. Follows the same
pattern as the existing walker.service.

Closes #2

defaults/user/systemd/user/nautilus.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=Nautilus File Manager (GApplication Service)
+After=graphical-session.target
+
+[Service]
+Type=simple
+ExecStart=nautilus --gapplication-service
+Restart=on-failure
+
+[Install]
+WantedBy=graphical-session.target