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/common.sh

@@ -0,0 +1,35 @@
+#!/bin/bash
+# Output helpers, prompts and small shared utilities.
+# Sourced by setup-gamescope-boot.sh; not meant to be run on its own.
+
+c_reset="\033[0m"; c_bold="\033[1m"; c_green="\033[32m"; c_yellow="\033[33m"; c_red="\033[31m"; c_cyan="\033[36m"
+
+info()  { echo -e "${c_cyan}[INFO]${c_reset} $*"; }
+ok()    { echo -e "${c_green}[OK]${c_reset} $*"; }
+warn()  { echo -e "${c_yellow}[WARN]${c_reset} $*"; }
+err()   { echo -e "${c_red}[ERROR] $*${c_reset}" >&2; }
+ask_yn() {
+    local prompt="$1" default="${2:-y}" reply
+    local hint="[Y/n]"; [[ "$default" == "n" ]] && hint="[y/N]"
+    read -rp "$(echo -e "${c_bold}${prompt}${c_reset} ${hint} ")" reply
+    reply="${reply:-$default}"
+    [[ "$reply" =~ ^[Yy]$ ]]
+}
+
+require_root_helper() {
+    # Re-exec any privileged step through sudo rather than requiring the
+    # whole script to run as root, so $HOME/whoami stay correct for the
+    # target user detection below.
+    if [[ $EUID -eq 0 && -z "${SUDO_USER:-}" ]]; then
+        err "Please run this script as your normal user (it will call sudo itself), not directly as root."
+        exit 1
+    fi
+}
+
+backup_file() {
+    local f="$1"
+    if [[ -f "$f" && ! -f "${f}.bak-gamescope-wizard" ]]; then
+        sudo cp -a "$f" "${f}.bak-gamescope-wizard"
+        info "Backed up $f -> ${f}.bak-gamescope-wizard"
+    fi
+}