feat: set default wallpaper on first KDE login via one-shot autostart

Add /etc/skel autostart that runs plasma-apply-wallpaperimage once on
first KDE login to set the Adwaita Morning wallpaper as default. Creates
a marker file so it never runs again. Works for both live ISO and
installed system.

iso-profiles/antergos/root-overlay/etc/skel/.config/autostart/antergos-wallpaper.desktop

@@ -0,0 +1,8 @@
+[Desktop Entry]
+Type=Application
+Name=Set Antergos NeXT Wallpaper
+Comment=Apply Antergos NeXT default wallpaper on first login
+Exec=/usr/local/bin/set-antergos-wallpaper.sh
+Terminal=false
+X-KDE-autostart-phase=2
+NoDisplay=true

iso-profiles/antergos/root-overlay/usr/local/bin/set-antergos-wallpaper.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+# One-shot wallpaper setter for Antergos NeXT
+# Runs at first login via autostart, then disables itself
+
+MARKER="$HOME/.config/antergos-wallpaper-set"
+
+if [[ -f "$MARKER" ]]; then
+  exit 0
+fi
+
+WALLPAPER="/usr/share/wallpapers/antergos-wallpaper/contents/images/adwaita-morning.webp"
+
+if [[ -f "$WALLPAPER" ]]; then
+  plasma-apply-wallpaperimage "$WALLPAPER" 2>/dev/null
+fi
+
+touch "$MARKER"