fix: stretch notifications widget to full width when enrich queue is empty

Grid only splits to two columns when both DashboardMentions and
DashboardNeedsMetadata have content to show.

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

frontend/src/routes/+page.svelte

@@ -100,7 +100,9 @@ $effect(() => {
 			</div>
 		{/if}
 
-		<div class="mt-6 grid gap-4 {(data.mentions?.length ?? 0) > 0 ? 'lg:grid-cols-2' : ''}">
+		<div
+			class="mt-6 grid gap-4 {(data.mentions?.length ?? 0) > 0 && (data.incompleteDocs?.length ?? 0) > 0 ? 'lg:grid-cols-2' : ''}"
+		>
 			<DashboardMentions mentions={data.mentions ?? []} />
 			<DashboardNeedsMetadata incompleteDocs={data.incompleteDocs ?? []} />
 		</div>