Make setup ISO-only

Remove legacy online installer entrypoints, collapse migrations for 4.0, and move setup responsibilities into target-side system, hardware, and user commands.

install/config/theme-system.sh

@@ -4,6 +4,7 @@ ln -snf /usr/share/icons/Adwaita/symbolic/actions/go-previous-symbolic.svg \
           /usr/share/icons/Yaru/scalable/actions/go-previous-symbolic.svg
 ln -snf /usr/share/icons/Adwaita/symbolic/actions/go-next-symbolic.svg \
           /usr/share/icons/Yaru/scalable/actions/go-next-symbolic.svg
+gtk-update-icon-cache /usr/share/icons/Yaru &>/dev/null || true
 
 # Chromium policy directory for theme
 mkdir -p /etc/chromium/policies/managed