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.

app/router.options.ts

@@ -0,0 +1,16 @@
+import type { RouterConfig } from "@nuxt/schema";
+
+export default <RouterConfig>{
+	scrollBehavior(to, from, savedPosition) {
+		if (savedPosition) {
+			return savedPosition;
+		}
+
+		// Product details → Product details: keep scroll position
+		if (from.path.startsWith("/details/") && to.path.startsWith("/details/")) {
+			return false;
+		}
+
+		return { top: 0, behavior: "instant" };
+	}
+};