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/SelectionBox.vue

@@ -0,0 +1,36 @@
+<template>
+	<NuxtLink
+		:to="path"
+		:title="ariaLabel"
+		:aria-label="ariaLabel"
+		:aria-checked="isActive"
+		:class="{
+			'border-black bg-white': isActive,
+			'border-transparent cursor-pointer bg-gray-100 opacity-80 hover:opacity-100 hover:border-gray-400 pointer': !isActive,
+			'opacity-30 cursor-not-allowed pointer-events-none': ariaDisabled
+		}"
+		class="flex flex-col items-center gap-y-4 rounded-md border-2 border-spacing-2 pt-2 pb-2 px-2 transition-all"
+	>
+		<div v-if="$slots.default" class="w-16 h-16 bg-transparent overflow-hidden">
+			<slot></slot>
+		</div>
+		<div
+			class="text-sm text-gray-800 pointer-events-none"
+			:class="{
+				'font-semibold': isActive
+			}"
+		>
+			{{ label }}
+		</div>
+	</NuxtLink>
+</template>
+
+<script setup lang="ts">
+defineProps<{
+	label: string;
+	path: string;
+	isActive?: boolean;
+	ariaLabel?: string;
+	ariaDisabled?: boolean;
+}>();
+</script>