refactor: move shared components to lib/shared/ sub-packages

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/shared/primitives/BackButton.svelte

@@ -0,0 +1,27 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+let { class: cls = 'mb-4', showLabel = true }: { class?: string; showLabel?: boolean } = $props();
+</script>
+
+<button
+	type="button"
+	onclick={() => history.back()}
+	aria-label={!showLabel ? m.btn_back() : undefined}
+	class="group {cls} inline-flex min-h-[44px] items-center text-xs font-bold tracking-widest text-ink-2 uppercase transition-colors outline-none hover:text-ink focus-visible:ring-2 focus-visible:ring-focus-ring"
+>
+	<svg
+		class="{showLabel ? 'mr-2' : ''} h-4 w-4 transform transition-transform group-hover:-translate-x-1"
+		fill="none"
+		stroke="currentColor"
+		viewBox="0 0 24 24"
+		aria-hidden="true"
+	>
+		<path
+			stroke-linecap="round"
+			stroke-linejoin="round"
+			stroke-width="2"
+			d="M10 19l-7-7m0 0l7-7m-7 7h18"
+		/>
+	</svg>
+	{#if showLabel}{m.btn_back()}{/if}
+</button>