fix: Add single user no password option with SDDM, fetch newest Vapor presets

- One wizard question: "single user, no password, like SteamOS" switches
  to SDDM (autologin via CachyOS's own steam-set-session, no workarounds)
  and removes lock screen, user switching and log out (launcher shows only
  Sleep/Restart/Shut Down; Meta+L and Ctrl+Alt+Del unbound). Declining
  keeps the current login manager and KDE's defaults
- Look up the newest steamdeck-kde-presets from Valve's release repos and
  verify its SHA-256; take over the new SteamOS 3.9 defaults
- Restart plasmashell through systemd so the desktop keeps the session's
  dark theme; drop the bundled icon in favour of Valve's and Steam's
- README and AGENTS.md updated

lib/common.sh

@@ -26,6 +26,18 @@ require_root_helper() {
     fi
 }
 
+plasma_applets() {
+    # Print "containment:applet" for every applet of plugin $1 in the
+    # user's Plasma layout, for kwriteconfig6 --group paths.
+    local conf="$HOME/.config/plasma-org.kde.plasma.desktop-appletsrc"
+    [[ -f "$conf" ]] || return 0
+    awk -v plugin="$1" '
+        /^\[Containments\]\[[0-9]+\]\[Applets\]\[[0-9]+\]$/ { split($0, p, /[][]+/); sect = p[3] ":" p[5] }
+        /^\[/ && !/\]\[Applets\]\[[0-9]+\]$/ { sect = "" }
+        $0 == "plugin=" plugin && sect != "" { print sect }
+    ' "$conf"
+}
+
 backup_file() {
     local f="$1"
     if [[ -f "$f" && ! -f "${f}.bak-gamescope-wizard" ]]; then