Wire theme restore/backup into bin/restore.sh and bin/backup.sh

- restore.sh: after folder+venv+frontend restore, runs
  assets/themes/install-theme.sh to rebuild the desktop wiring
- backup.sh: snapshots live xfconf + GTK 3/4 settings.ini into
  assets/themes/restore-snapshot/ before the router mirror, so the
  backup captures actual state (install-theme.sh stays the applier)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

assets/themes/restore-snapshot/gtk-4.0-settings.ini

@@ -0,0 +1,12 @@
+[Settings]
+gtk-application-prefer-dark-theme=true
+gtk-theme-name=NexusOS
+gtk-cursor-theme-name=DMZ-White
+gtk-cursor-theme-size=24
+gtk-decoration-layout=icon:minimize,maximize,close
+gtk-enable-animations=true
+gtk-font-name=Ubuntu 10
+gtk-icon-theme-name=NexusOS
+gtk-modules=colorreload-gtk-module:window-decorations-gtk-module
+gtk-primary-button-warps-slider=false
+gtk-xft-dpi=98304