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

setup-gamescope-boot.sh

@@ -80,18 +80,20 @@ fi
 ok "Using user: $TARGET_USER"
 echo
 
-check_display_manager
+choose_setup_mode
 echo
 
 install_required_packages
 echo
 
-configure_autologin
-install_session_sync
+setup_login_manager
 
 setup_steam_desktop
 echo
 
+setup_single_user
+echo
+
 setup_led_driver
 
 setup_vapor_theme
@@ -105,16 +107,21 @@ echo
 echo -e "${c_bold}Setup complete.${c_reset}"
 echo "What this did:"
 echo "  - Installed gamescope-session-cachyos, steam, mangohud and friends (if missing)"
-echo "  - Created /etc/plasmalogin.conf.d (fixes Switch-to-Desktop crash)"
-echo "  - Set $BASE_CONF to autologin '$TARGET_USER' into gamescope, with Relogin=true"
-echo "  - Installed a sync bridge + systemd watcher so Steam's Switch-to-Desktop"
-echo "    (and cachyos-gamescope-autologin.service resetting back to gamescope"
-echo "    on logout) both actually take effect"
+if [[ "$LOGIN_MANAGER" == "sddm" ]]; then
+    echo "  - Made SDDM the login manager, autologging '$TARGET_USER' into gamescope"
+    echo "    with Relogin=true, like SteamOS (active from the next boot)"
+else
+    echo "  - Created /etc/plasmalogin.conf.d (fixes Switch-to-Desktop crash)"
+    echo "  - Set $BASE_CONF to autologin '$TARGET_USER' into gamescope, with Relogin=true"
+    echo "  - Installed a sync bridge + systemd watcher so Steam's Switch-to-Desktop"
+    echo "    (and cachyos-gamescope-autologin.service resetting back to gamescope"
+    echo "    on logout) both actually take effect"
+    echo "  - Added /etc/sudoers.d/gamescope-session-switch so the desktop shortcut works"
+fi
 echo "  - Configured permanent silent Steam autostart so Steam+X works everywhere"
 echo "  - Injected -steamos3 flag to force original Steam Deck overlay glyphs"
 echo "  - Optionally installed Valve's Vapor (Steam Deck) KDE theme, if you chose to"
 echo "  - On Valve Fremont hardware, optionally set up the leds-valve front LED bar driver"
-echo "  - Added /etc/sudoers.d/gamescope-session-switch so the desktop shortcut works"
 echo
 echo "Backups of any files this script modified were saved with a"
 echo ".bak-gamescope-wizard suffix next to the original."