feat: extract shop from mp/shop — initial libreshop/shop

Source moved verbatim from mp/shop/ on 2026-04-29; mp was the first
concrete adapter consuming the libreshop toolkit. Builds and publishes
git.librete.ch/libreshop/shop on every main / v* push via the standard
.gitea/workflows/build.yml shared across libreshop components.

utils/scrollProgress.ts

@@ -0,0 +1,8 @@
+export const scrollProgress = () => {
+	if (!import.meta.client) return 0;
+
+	const { documentElement, body } = document;
+	const windowScroll = body.scrollTop || documentElement.scrollTop;
+	const height = documentElement.scrollHeight - documentElement.clientHeight;
+	return height > 0 ? Math.round((windowScroll / height) * 100) : 0;
+};