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/desktop-shortcut.sh

@@ -17,21 +17,29 @@ create_desktop_shortcut() {
 
     mkdir -p "$desktop_dir"
 
-    # Let the shortcut restart the login manager without a password prompt
-    # (it runs with Terminal=false, so sudo can't ask). Scoped to exactly
-    # the two commands the shortcut needs, for this user only.
-    local sudoers_tmp
-    sudoers_tmp="$(mktemp)"
-    cat > "$sudoers_tmp" << EOF
+    # On SDDM, steamos-session-select logging out is enough: Relogin=true
+    # logs straight back in to the newly selected session, like SteamOS.
+    # plasma-login-manager needs the sync bridge run and a restart, which
+    # the shortcut can't sudo for interactively (Terminal=false), so it gets
+    # a password-less rule scoped to exactly those two commands.
+    local exec_line="steamos-session-select gamescope"
+    if [[ "$LOGIN_MANAGER" == "plasmalogin" ]]; then
+        exec_line="sh -c 'steamos-session-select gamescope && sudo -n /usr/bin/systemctl start sync-steamos-session.service && sudo -n /usr/bin/systemctl restart plasmalogin'"
+        local sudoers_tmp
+        sudoers_tmp="$(mktemp)"
+        cat > "$sudoers_tmp" << EOF
 $TARGET_USER ALL=(root) NOPASSWD: /usr/bin/systemctl start sync-steamos-session.service, /usr/bin/systemctl restart plasmalogin
 EOF
-    if sudo visudo -cf "$sudoers_tmp" >/dev/null; then
-        sudo install -m 0440 -o root -g root "$sudoers_tmp" /etc/sudoers.d/gamescope-session-switch
-        ok "Installed /etc/sudoers.d/gamescope-session-switch for the shortcut."
+        if sudo visudo -cf "$sudoers_tmp" >/dev/null; then
+            sudo install -m 0440 -o root -g root "$sudoers_tmp" /etc/sudoers.d/gamescope-session-switch
+            ok "Installed /etc/sudoers.d/gamescope-session-switch for the shortcut."
+        else
+            warn "Generated sudoers rule failed validation; the shortcut will not be able to restart plasmalogin."
+        fi
+        rm -f "$sudoers_tmp"
     else
-        warn "Generated sudoers rule failed validation; the shortcut will not be able to restart plasmalogin."
+        sudo rm -f /etc/sudoers.d/gamescope-session-switch
     fi
-    rm -f "$sudoers_tmp"
 
     # SteamOS itself uses Valve's "gaming-return" icon (Steam logo with a
     # return arrow; the Deck-style arrow is only used on Steam Deck
@@ -48,7 +56,7 @@ EOF
 [Desktop Entry]
 Name=Return to Gaming Mode
 Comment=Switch session back to Gamescope
-Exec=sh -c 'steamos-session-select gamescope && sudo -n /usr/bin/systemctl start sync-steamos-session.service && sudo -n /usr/bin/systemctl restart plasmalogin'
+Exec=$exec_line
 Icon=$shortcut_icon
 Terminal=false
 Type=Application