feat(quickshell): run it as a systemd user service

spawn-at-startup gave it no restart, so a crash left the session without a bar
until the next login — which is what the Alt+W toggle used to paper over.

defaults/etc/systemd/user/quickshell.service

@@ -0,0 +1,16 @@
+# ABOUTME: systemd user service for the Quickshell bar, launcher and popouts.
+# ABOUTME: Replaces the spawn-at-startup in niri's config.kdl, which never restarted it.
+
+[Unit]
+Description=Quickshell bar and shell
+PartOf=graphical-session.target
+After=graphical-session.target
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/quickshell -c moonarch
+Restart=on-failure
+RestartSec=3
+
+[Install]
+WantedBy=graphical-session.target

defaults/xdg/niri/config.kdl

@@ -94,8 +94,8 @@ layout {
 
 // xwayland-satellite is managed automatically since niri 25.08
 // kanshi is managed via systemd user service (kanshi.service)
-// Bar: quickshell is the default; Waybar stays installed as a reserve (toggle with Alt+W).
-spawn-at-startup "quickshell" "-c" "moonarch"
+// quickshell is managed via systemd user service (quickshell.service), which
+// restarts it on a crash — spawn-at-startup left the session without a bar.
 // swaync disabled — quickshell owns notifications now; package kept installed as a reserve.
 // spawn-at-startup "swaync"
 spawn-at-startup "/usr/lib/polkit-gnome/polkit-gnome-authentication-agent-1"