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.

components/HeaderNavLink.vue

@@ -0,0 +1,21 @@
+<template>
+	<NuxtLink
+		:to="path"
+		class="px-4 py-0.5 rounded-full font-medium text-lg text-black transition-all duration-200 flex flex-col justify-center hover:bg-gray-900 hover:text-white"
+		active-class="!bg-gray-900 !text-white"
+		aria-label="Navigation"
+		:title="description ?? label"
+	>
+		{{ label }}
+	</NuxtLink>
+</template>
+
+<script setup lang="ts">
+defineProps<{
+	path?: string;
+	to?: string;
+	label: string;
+	description?: string;
+	isDarkMode?: boolean;
+}>();
+</script>