i18n(briefwechsel): ThumbnailRow direction label via Paraglide

Adds row_direction_sent / row_direction_received keys across the
three locale files (de: Gesendet/Empfangen, en: Sent/Received, es:
Enviada/Recibida) and routes ThumbnailRow's directionLabel through
Paraglide. An English or Spanish screen-reader user now hears
"Sent:" / "Enviada:" in their language, matching the DistributionBar
i18n pass.

Refs #305
Fixes @leonievoss round-2 follow-up from PR review

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

frontend/src/lib/components/ThumbnailRow.svelte

@@ -3,6 +3,7 @@ import ConversationThumbnail from '$lib/components/ConversationThumbnail.svelte'
 import TagChipList from '$lib/components/TagChipList.svelte';
 import { formatDate } from '$lib/utils/date';
 import { relativeYearsDe } from '$lib/relativeTime';
+import * as m from '$lib/paraglide/messages.js';
 
 type Person = { id: string; firstName?: string | null; lastName: string; displayName: string };
 type Tag = { id: string; name: string };
@@ -47,7 +48,7 @@ const otherPartyName = $derived(
 const relativeYearLabel = $derived(
 	doc.documentDate ? relativeYearsDe(new Date(doc.documentDate + 'T12:00:00'), now) : ''
 );
-const directionLabel = $derived(isOut ? 'Gesendet' : 'Empfangen');
+const directionLabel = $derived(isOut ? m.row_direction_sent() : m.row_direction_received());
 const ariaLabel = $derived(
 	`${directionLabel}: ${title}${doc.documentDate ? `, ${formatDate(doc.documentDate)}` : ''}`
 );