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/slugify.ts

@@ -0,0 +1,9 @@
+export function slugify(text: string): string {
+	return text
+		.toLowerCase()
+		.replace(/\s+/g, "-")
+		.replace(/ü/g, "ue")
+		.replace(/ä/g, "ae")
+		.replace(/ö/g, "oe")
+		.replace(/ß/g, "ss");
+}