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

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

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

@@ -0,0 +1,15 @@
+<script lang="ts">
+let { label }: { label: string } = $props();
+</script>
+
+<div
+	data-testid="group-divider"
+	role="separator"
+	aria-label={label}
+	class="relative flex items-center py-2 text-center"
+>
+	<div class="flex-grow border-t border-line"></div>
+	<span class="mx-4 font-sans text-sm font-bold tracking-widest text-ink/60 uppercase">{label}</span
+	>
+	<div class="flex-grow border-t border-line"></div>
+</div>