Restructure for bare Fedora: lib/ + modules/, DE selection restored

Splits setup.sh into lib/common.sh, lib/session.sh and seven modules
(3 DE, 4 TWM). Targets a bare Fedora 44+ install.

Fixes carried over from the first build:
- DEBUG_MODE was unbound under set -u, aborting the script at the version banner
- mangled 'echohttps://...' line left over from the starburst split
- set -e was missing entirely; --debug flag parsing restored
- unguarded 'clear' aborted the run without TERM
- $PRETTY_NAME unguarded under set -u
- modules were sourced, leaking shell options into the parent
- Flathub added twice (system-wide + per-user)
- no dnf5-plugins check before using 'dnf config-manager'
- no Fedora release gate
- /usr/share/xsessions assumed to exist when writing session entries

modules/de/gnome.sh

@@ -0,0 +1,29 @@
+#!/bin/bash
+set -uo pipefail
+
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+# shellcheck source=../../lib/common.sh
+source "$HERE/../../lib/common.sh"
+# shellcheck source=../../lib/session.sh
+source "$HERE/../../lib/session.sh"
+
+if is_debug; then set +e; else set -e; fi
+
+step "Desktop environment: GNOME"
+rule
+
+if pkg_installed gnome-shell; then
+    ok "GNOME already installed."
+else
+    info "Installing the GNOME desktop..."
+    # Group names have shifted across releases; take whichever exists.
+    install_group_any gnome-desktop gnome-desktop-environment workstation-product-environment
+    install_optional gnome-tweaks gnome-extensions-app
+fi
+
+# GNOME ships and enables gdm itself. We deliberately leave it alone: a DE module
+# never fights the DE over its own greeter. A TWM module switched on top will
+# move the system to SDDM instead.
+info "Current display manager: $(detect_display_manager)"
+
+ok "GNOME setup complete."