feat(briefwechsel): ThumbnailRow aria-label leads with Gesendet/Empfangen

Without this prefix, a color-blind user or screen-reader user has no
indication of correspondence direction — the colored left border is
information but not announced, and the arrow glyphs were removed in
the earlier layout pass. Prepending "Gesendet:" or "Empfangen:" to
the aria-label gives assistive-tech users the direction first so the
row identity is unambiguous even without color perception.

Refs #305
Fixes @leonievoss WCAG 1.4.1 concern from PR review

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

frontend/src/lib/components/ThumbnailRow.svelte

@@ -50,8 +50,9 @@ const relativeYearLabel = $derived(
 		? relativeYearsDe(new Date(doc.documentDate + 'T12:00:00'), now ?? new Date())
 		: ''
 );
+const directionLabel = $derived(isOut ? 'Gesendet' : 'Empfangen');
 const ariaLabel = $derived(
-	`${title}${doc.documentDate ? `, ${formatDate(doc.documentDate)}` : ''}`
+	`${directionLabel}: ${title}${doc.documentDate ? `, ${formatDate(doc.documentDate)}` : ''}`
 );
 </script>