fix: Replace the question flow with a menu that turns components on and off

- Menu detects what is on and applies only the changes: SteamOS
  conversion, SteamOS theme, Steam Deck/Machine icons, single user mode,
  and Steam Machine support (only on Fremont hardware)
- Everything is reversible: KDE settings go through an undo journal
  (lib/state.sh), system files are restored from their backups
- Steam Machine support: LED driver, udev rule giving the user the LED
  files, steamos-manager for Steam's hardware settings; OpenRGB dropped
- Edit Plasma files only while plasmashell is stopped so it can't
  overwrite them
- README and AGENTS.md describe the menu, the component model and the
  test environment

lib/single-user.sh

@@ -0,0 +1,68 @@
+#!/bin/bash
+# Single user, no password, like SteamOS: no lock screen, user switching or
+# log out, since typing a password with a controller is miserable. Goes
+# together with SDDM (see lib/login-manager.sh). All changes are journaled
+# per user (lib/state.sh), so turning it off restores KDE's previous state.
+# Sourced by setup-gamescope-boot.sh; not meant to be run on its own.
+
+single_status() {
+    [[ "$(kreadconfig6 --file kdeglobals --group "KDE Action Restrictions" --key action/lock_screen)" == false ]]
+}
+
+single_enable() {
+    info "Turning off the lock screen, user switching and logging out..."
+    stop_plasmashell_for_edit
+
+    # Hides Lock / Switch User in menus.
+    local action applet
+    for action in lock_screen switch_user start_new_session; do
+        kset single kdeglobals "KDE Action Restrictions" "action/$action" false
+    done
+    # Never lock on idle or resume.
+    kset single kscreenlockerrc Daemon Autolock false
+    kset single kscreenlockerrc Daemon LockOnResume false
+    kset single kscreenlockerrc Daemon Timeout 0
+    # The restrictions don't cover the shortcuts: unbind Meta+L and the
+    # Ctrl+Alt+Del logout screen (format: current,default,description).
+    kset single kglobalshortcutsrc ksmserver "Lock Session" $'none,Screensaver\tMeta+L,Lock Session'
+    kset single kglobalshortcutsrc ksmserver "Log Out" 'none,Ctrl+Alt+Del,Show Logout Screen'
+    # Launcher: only Sleep / Restart / Shut Down, no Session dropdown (where
+    # Log Out lives). Restricting action/logout instead would also hide
+    # Restart and Shut Down.
+    for applet in $(plasma_applets org.kde.plasma.kickoff); do
+        local grp="Containments|${applet%%:*}|Applets|${applet#*:}|Configuration|General"
+        kset single plasma-org.kde.plasma.desktop-appletsrc "$grp" primaryActions 3
+        kset single plasma-org.kde.plasma.desktop-appletsrc "$grp" systemFavorites 'suspend,reboot,shutdown'
+    done
+
+    restart_plasmashell_if_stopped
+    ok "Single user: no lock screen, user switching or log out."
+}
+
+single_disable() {
+    info "Restoring the lock screen, user switching and logging out..."
+    stop_plasmashell_for_edit
+    if has_journal single; then
+        krevert single
+    else
+        # Set up by a version of this script without the undo journal:
+        # fall back to KDE's defaults for everything it changes.
+        local action applet key
+        for action in lock_screen switch_user start_new_session; do
+            kwriteconfig6 --file kdeglobals --group "KDE Action Restrictions" --key "action/$action" --delete
+        done
+        for key in Autolock LockOnResume Timeout; do
+            kwriteconfig6 --file kscreenlockerrc --group Daemon --key "$key" --delete
+        done
+        kwriteconfig6 --file kglobalshortcutsrc --group ksmserver --key "Lock Session" $'Screensaver\tMeta+L,Screensaver\tMeta+L,Lock Session'
+        kwriteconfig6 --file kglobalshortcutsrc --group ksmserver --key "Log Out" 'Ctrl+Alt+Del,Ctrl+Alt+Del,Show Logout Screen'
+        for applet in $(plasma_applets org.kde.plasma.kickoff); do
+            kwriteconfig6 --file plasma-org.kde.plasma.desktop-appletsrc --group Containments --group "${applet%%:*}" \
+                --group Applets --group "${applet#*:}" --group Configuration --group General --key primaryActions --delete
+            kwriteconfig6 --file plasma-org.kde.plasma.desktop-appletsrc --group Containments --group "${applet%%:*}" \
+                --group Applets --group "${applet#*:}" --group Configuration --group General --key systemFavorites --delete
+        done
+    fi
+    restart_plasmashell_if_stopped
+    ok "KDE's normal lock screen and user switching are back."
+}