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/packages.sh

@@ -0,0 +1,83 @@
+#!/bin/bash
+# Required packages and AUR helper handling.
+# Sourced by setup-gamescope-boot.sh; not meant to be run on its own.
+
+aur_noninteractive_flags() {
+    # Fully non-interactive install flags for the given AUR helper: no
+    # PKGBUILD diff/edit/cleanbuild prompts, no provider-selection prompts
+    # (e.g. picking kernel headers for DKMS), and makepkg itself silenced.
+    case "$1" in
+        yay)
+            echo --needed --noconfirm --answerclean None --answerdiff None \
+                 --answeredit None --mflags "--noconfirm"
+            ;;
+        paru)
+            echo --needed --noconfirm --skipreview --mflags "--noconfirm"
+            ;;
+    esac
+}
+
+find_aur_helper() {
+    if command -v yay >/dev/null 2>&1; then
+        echo yay
+    elif command -v paru >/dev/null 2>&1; then
+        echo paru
+    fi
+}
+
+bootstrap_yay() {
+    # Builds and installs yay from the AUR using only makepkg + base-devel
+    # (both from the official repos), so we have an AUR helper available
+    # without assuming the user already set one up.
+    info "Installing base-devel and git (needed to build an AUR helper)..."
+    sudo pacman -S --needed --noconfirm base-devel git || return 1
+
+    local tmp_dir
+    tmp_dir="$(mktemp -d)"
+    info "Cloning yay-bin from the AUR..."
+    if ! git clone --depth 1 https://aur.archlinux.org/yay-bin.git "$tmp_dir/yay-bin"; then
+        err "Failed to clone yay-bin from the AUR."
+        rm -rf "$tmp_dir"
+        return 1
+    fi
+
+    info "Building and installing yay (this runs makepkg as your user, not root)..."
+    if (cd "$tmp_dir/yay-bin" && makepkg -si --noconfirm); then
+        ok "yay installed."
+        rm -rf "$tmp_dir"
+        return 0
+    else
+        err "Building yay failed. See errors above."
+        rm -rf "$tmp_dir"
+        return 1
+    fi
+}
+
+install_required_packages() {
+    # Step 2: gamescope/Steam and the desktop helpers the rest of the setup uses.
+    local -a MISSING
+    info "Checking required packages: gamescope-session-cachyos, steam, mangohud, xterm, ttf-liberation, wqy-zenhei, plasma-keyboard"
+    mapfile -t MISSING < <(pacman -T gamescope-session-cachyos steam mangohud xterm ttf-liberation wqy-zenhei plasma-keyboard)
+
+    if [[ ${#MISSING[@]} -gt 0 ]]; then
+        info "Missing packages: ${MISSING[*]}"
+        if ask_yn "Install them now?"; then
+            sudo pacman -S --needed --noconfirm "${MISSING[@]}" || { err "Package installation failed."; exit 1; }
+        else
+            err "Cannot continue without these packages."
+            exit 1
+        fi
+    else
+        ok "All required packages already installed."
+    fi
+}
+
+ensure_aur_helper() {
+    # yay or paru, bootstrapping yay from the AUR when neither is installed.
+    if [[ -n "$(find_aur_helper)" ]]; then
+        ok "AUR helper already present."
+    else
+        info "No AUR helper (yay/paru) found. Installing yay..."
+        bootstrap_yay
+    fi
+}