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/CheckoutError.vue

@@ -0,0 +1,45 @@
+<template>
+	<div class="bg-red-50 border border-red-200 rounded-xl p-6 text-center max-w-md mx-auto">
+		<div class="inline-flex items-center justify-center w-12 h-12 rounded-full bg-red-100 mb-4">
+			<svg class="w-6 h-6 text-red-600" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+				<path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 9v2m0 4h.01m-6.938 4h13.856c1.54 0 2.502-1.667 1.732-3L13.732 4c-.77-1.333-2.694-1.333-3.464 0L3.34 16c-.77 1.333.192 3 1.732 3z" />
+			</svg>
+		</div>
+		<h3 class="text-lg font-semibold text-red-800 mb-2">{{ title }}</h3>
+		<p class="text-red-600 mb-6">{{ message }}</p>
+		<div class="flex flex-col sm:flex-row gap-3 justify-center">
+			<button
+				v-if="showRetry"
+				@click="$emit('retry')"
+				class="px-6 py-2.5 bg-red-600 text-white rounded-full font-medium hover:bg-red-700 transition-colors"
+			>
+				Erneut versuchen
+			</button>
+			<NuxtLink
+				to="/"
+				class="px-6 py-2.5 bg-white border border-red-200 text-red-700 rounded-full font-medium hover:bg-red-50 transition-colors"
+			>
+				Zur Startseite
+			</NuxtLink>
+		</div>
+	</div>
+</template>
+
+<script setup lang="ts">
+withDefaults(
+	defineProps<{
+		title?: string;
+		message?: string;
+		showRetry?: boolean;
+	}>(),
+	{
+		title: "Ein Fehler ist aufgetreten",
+		message: "Dein Warenkorb konnte nicht geladen werden. Bitte versuche es erneut.",
+		showRetry: true
+	}
+);
+
+defineEmits<{
+	retry: [];
+}>();
+</script>