LXDE: add Custom packages option and respect existing display manager

- New 'LXDE Custom (Choose packages)' menu entry: fixed base
  (lxde-core + xserver-xorg, lightdm only when no display manager
  is configured) plus a data-driven checklist of 16 extras.
- lxterminal, polkitd, notification-daemon and light-locker stay out
  of the checklist: they arrive via lxde-core Recommends (verified
  against packages.debian.org/trixie, 2026-10-16).
- Full/Core/Custom now honor an already-configured display manager
  via _existing_display_manager (GNOME/KDE pattern): no debconf
  preseed, no lightdm install, no enable over an existing DM.
- Extract _select_packages_checklist helper to utils.sh; XFCE
  Custom consumes it with no functional change.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

modules/utils.sh

@@ -805,6 +805,28 @@ _confirm_install_list() {
     return 1
 }
 
+# Show a multi-select checklist of packages and return the
+# selection as a space-separated string ("" on cancel/Esc).
+# Args: title prompt "pkg|description" [pkg|description ...]
+# Already-installed packages are pre-checked via _state.
+_select_packages_checklist() {
+    local title="$1" prompt="$2"
+    shift 2
+    local -a items=() entry
+    for entry in "$@"; do
+        local pkg="${entry%%|*}"
+        items+=("$pkg" "${entry#*|}" "$(_state "$pkg")")
+    done
+    local choices
+    # _checklist swallows whiptail's cancel (Esc) exit code
+    # via `|| true`, so a cancelled dialog looks like an empty
+    # selection here; an empty result means "nothing to install".
+    choices=$(_checklist "$title" "$prompt" \
+        $TUI_ALTO $TUI_ANCHO $TUI_ALTO_LISTA "${items[@]}")
+    choices=$(echo "$choices" | tr -d '"')
+    echo "$choices"
+}
+
 # Install a package with confirmation prompt.
 # Handles set -e: failures are caught and reported, not fatal.
 _install_pkg() {