Fill desktop gaps: GUI apps, printing, media, Qt/portal integration

Add the packages a general desktop is expected to ship, chosen to stay
opinionated but average-user friendly:

- Editors: neovim (+ ripgrep, fd for a usable nvim/fzf experience)
- GUI basics: gnome-text-editor, gnome-calculator, file-roller, loupe
- Media: vlc (BOS had codecs but no player)
- Hardware: cups + cups-pk-helper + system-config-printer (enable
  cups.socket in post-install), blueman, seahorse
- Platform: qt5-wayland + qt6-wayland (native Wayland for Qt apps under
  the QT_QPA_PLATFORM=wayland we set), xdg-desktop-portal-gtk (file
  dialogs/screenshare for Flatpak/Electron/Zen), flatpak

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

iso/airootfs/etc/calamares/post-install.sh

@@ -100,7 +100,7 @@ fi
 # ---------------------------------------------------------------------------
 for unit in NetworkManager.service bluetooth.service systemd-timesyncd.service \
             tlp.service greetd.service snapper-cleanup.timer grub-btrfsd.service \
-            fstrim.timer; do
+            fstrim.timer cups.socket; do
     systemctl enable "$unit" || echo "WARN: failed to enable $unit"
 done
 systemctl set-default graphical.target || echo "WARN: set-default graphical failed"