fix: Fix setup bugs, add SteamOS desktop look and LED driver, split into modules

- Fix session switching: sync bridge only edits [Autologin], sync service
  no longer hits systemd's start limit, re-runs don't duplicate settings
- Make the Return to Gaming Mode shortcut work without a password prompt
- Steam desktop autostart only runs in Plasma, not inside gamescope
- Vapor theme: fix install path, wallpaper, icon and color scheme; take
  over Valve's SteamOS defaults, GTK theme, panel layout and dark mode;
  always fetch the newest steamdeck-kde-presets with checksum verification
- Steam Machine: install kernel headers and build/load the leds-valve
  driver reliably; ask before installing OpenRGB
- Split the script into lib/ modules per responsibility
- Rewrite README for end users with a technical details section; add
  AGENTS.md

lib/desktop-shortcut.sh

@@ -0,0 +1,62 @@
+#!/bin/bash
+# "Return to Gaming Mode" desktop shortcut and the sudoers rule it needs.
+# Sourced by setup-gamescope-boot.sh; not meant to be run on its own.
+
+create_desktop_shortcut() {
+    # Determine the home directory of the target user
+    local user_home
+    user_home="$(getent passwd "$TARGET_USER" | cut -d: -f6)"
+
+    # Locate the correct Desktop directory
+    local desktop_dir
+    if [[ -f "$user_home/.config/user-dirs.dirs" ]]; then
+        desktop_dir=$(grep '^XDG_DESKTOP_DIR=' "$user_home/.config/user-dirs.dirs" | cut -d '"' -f 2)
+        desktop_dir="${desktop_dir/\$HOME/$user_home}"
+    fi
+    desktop_dir="${desktop_dir:-$user_home/Desktop}"
+
+    mkdir -p "$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
+$TARGET_USER ALL=(root) NOPASSWD: /usr/bin/systemctl start sync-steamos-session.service, /usr/bin/systemctl restart plasmalogin
+EOF
+    if sudo visudo -cf "$sudoers_tmp" >/dev/null; then
+        sudo install -m 0440 -o root -g root "$sudoers_tmp" /etc/sudoers.d/gamescope-session-switch
+        ok "Installed /etc/sudoers.d/gamescope-session-switch for the shortcut."
+    else
+        warn "Generated sudoers rule failed validation; the shortcut will not be able to restart plasmalogin."
+    fi
+    rm -f "$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
+    # hardware). It comes with the Vapor theme; without it, use Steam's icon.
+    local shortcut_icon="steam"
+    [[ -f "$user_home/.local/share/icons/hicolor/scalable/actions/gaming-return.svg" ]] &&
+        shortcut_icon="gaming-return"
+
+    # 2. Generate the .desktop shortcut with instant session switcher strings
+    local shortcut_path="$desktop_dir/Return to Gaming Mode.desktop"
+    info "Creating 'Return to Gaming Mode' desktop shortcut at: $shortcut_path"
+
+    cat << EOF > "$shortcut_path"
+[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'
+Icon=$shortcut_icon
+Terminal=false
+Type=Application
+Categories=System;
+EOF
+
+    # Fix permissions for the target user and KDE Plasma desktop ecosystem
+    chmod +x "$shortcut_path"
+
+    ok "Desktop shortcut created successfully."
+}