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."

modules/de/plasma.sh

@@ -0,0 +1,26 @@
+#!/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: KDE Plasma"
+rule
+
+if pkg_installed plasma-workspace; then
+    ok "Plasma already installed."
+else
+    info "Installing the KDE Plasma desktop..."
+    install_group_any kde-desktop kde-desktop-environment
+fi
+
+# Plasma's native greeter is SDDM; make sure it is actually the active one.
+install_packages sddm
+switch_display_manager sddm
+
+ok "Plasma setup complete."

modules/de/xfce.sh

@@ -0,0 +1,50 @@
+#!/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 "$HERE/../../lib/session.sh"
+
+if is_debug; then set +e; else set -e; fi
+
+step "Desktop environment: XFCE"
+rule
+
+if pkg_installed xfce4-session; then
+    ok "XFCE already installed."
+else
+    info "Installing the XFCE desktop..."
+    install_group_any xfce-desktop xfce-desktop-environment
+    install_optional "@xfce-extra-plugins"
+fi
+
+# XFCE is X11. It needs an X server present even when it is only the session
+# provider for chadwm.
+install_packages xorg-x11-server-Xorg xorg-x11-xinit
+
+# XFCE's native greeter is lightdm
+if ! pkg_installed lightdm; then
+    install_packages lightdm
+fi
+switch_display_manager lightdm
+
+install_nerd_font JetBrainsMono
+install_packages alacritty dconf
+
+# Point XFCE's terminal preference at alacritty, seeding the config directly so
+# it applies on first login rather than needing a session restart.
+XFCONF_DIR="$HOME/.config/xfce4/xfconf/xfce-perchannel-xml"
+mkdir -p "$XFCONF_DIR"
+info "Seeding XFCE session defaults..."
+cat > "$XFCONF_DIR/xfce4-session.xml" <<'EOF'
+<?xml version="1.0" encoding="UTF-8"?>
+<channel name="xfce4-session" version="1.0">
+  <property name="general" type="empty">
+    <property name="TerminalEmulator" type="string" value="alacritty"/>
+  </property>
+</channel>
+EOF
+
+ok "XFCE setup complete."