feat(#145): add DashboardNeedsMetadata widget component

Shows documents with missing metadata as a dashboard widget with links
to the enrich workflow. Renders nothing when the list is empty.

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

frontend/src/lib/components/DashboardNeedsMetadata.svelte

@@ -0,0 +1,38 @@
+<script lang="ts">
+type IncompleteDocumentDTO = {
+	id: string;
+	title: string;
+};
+
+interface Props {
+	incompleteDocs: IncompleteDocumentDTO[];
+}
+
+let { incompleteDocs }: Props = $props();
+</script>
+
+{#if incompleteDocs.length > 0}
+	<div
+		data-testid="dashboard-needs-metadata"
+		class="border-brand-sand rounded-sm border bg-white p-6"
+	>
+		<h2 class="mb-4 font-sans text-xs font-bold tracking-widest text-gray-400 uppercase">
+			Metadaten fehlen
+		</h2>
+		{#each incompleteDocs as doc (doc.id)}
+			<div class="border-brand-sand flex items-center border-b py-2 last:border-0">
+				<a
+					href="/enrich/{doc.id}"
+					class="font-serif text-sm text-ink hover:text-brand-navy hover:underline"
+				>
+					{doc.title}
+				</a>
+			</div>
+		{/each}
+		<div class="mt-4">
+			<a href="/enrich" class="font-sans text-xs text-brand-navy/60 hover:text-brand-navy">
+				Alle anzeigen
+			</a>
+		</div>
+	</div>
+{/if}