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.

pages/checkout/index.vue

@@ -0,0 +1,8 @@
+<script setup lang="ts">
+// Redirect /checkout to /checkout/1
+definePageMeta({
+	redirect: "/checkout/1"
+});
+
+navigateTo("/checkout/1", { replace: true });
+</script>