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.spec.ts

@@ -127,7 +127,7 @@ describe('ThumbnailRow', () => {
 		expect(link.className).toContain('border-l-accent');
 	});
 
-	it('exposes a descriptive aria-label combining title and date', () => {
+	it('exposes a descriptive aria-label combining direction, title, and date', () => {
 		render(ThumbnailRow, {
 			doc: baseDoc,
 			isOut: true,
@@ -137,10 +137,23 @@ describe('ThumbnailRow', () => {
 
 		const link = document.querySelector('a[href="/documents/d1"]') as HTMLElement;
 		const label = link.getAttribute('aria-label') ?? '';
+		expect(label).toMatch(/^Gesendet:/);
 		expect(label).toContain('Liebe Anna');
 		expect(label).toMatch(/1950/);
 	});
 
+	it('aria-label begins with "Empfangen:" for incoming letters', () => {
+		render(ThumbnailRow, {
+			doc: baseDoc,
+			isOut: false,
+			showOtherParty: false,
+			now: new Date('2026-06-01T00:00:00Z')
+		});
+
+		const link = document.querySelector('a[href="/documents/d1"]') as HTMLElement;
+		expect(link.getAttribute('aria-label') ?? '').toMatch(/^Empfangen:/);
+	});
+
 	it('does not inject raw HTML when summary contains markup (XSS regression)', () => {
 		render(ThumbnailRow, {
 			doc: {