fix: replace hardcoded bg-white/border-brand-sand/text-brand-navy with semantic tokens in dashboard widgets

All four dashboard components (ResumeStrip, Mentions, NeedsMetadata, RecentDocuments)
used static brand colors that do not adapt to dark mode. Replace with bg-surface,
border-line, text-ink, text-ink-2 throughout.

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

frontend/src/lib/components/DashboardMentions.svelte

@@ -16,16 +16,16 @@ let { mentions }: Props = $props();
 </script>
 
 {#if mentions.length > 0}
-	<div data-testid="dashboard-mentions" class="border-brand-sand rounded-sm border bg-white p-6">
+	<div data-testid="dashboard-mentions" class="rounded-sm border border-line bg-surface p-6">
 		<h2 class="mb-4 font-sans text-xs font-bold tracking-widest text-gray-400 uppercase">
 			Erwähnungen
 		</h2>
 		{#each mentions as mention (mention.id)}
-			<div class="border-brand-sand flex items-center gap-3 border-b py-2 last:border-0">
+			<div class="flex items-center gap-3 border-b border-line py-2 last:border-0">
 				{#if mention.documentId}
 					<a
 						href="/documents/{mention.documentId}"
-						class="font-serif text-sm text-ink hover:text-brand-navy"
+						class="font-serif text-sm text-ink hover:text-ink-2"
 					>
 						{mention.actorName ?? ''}
 					</a>