feat(timeline): add LetterCard component

Single archive letter: sender → receiver (Unbekannt fallback for empty names,
REQ-014), title, precision date chip via timelineDateLabel (omitted when null,
REQ-013), linking to exactly /documents/{documentId} with no target (REQ-023).
44px touch target enforced inline + focus-visible ring (REQ-020). OCR/import
text via {...} escaping + whitespace-pre-line, no {@html} (REQ-021).

Refs #779
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/timeline/LetterCard.svelte

@@ -0,0 +1,42 @@
+<script lang="ts">
+import * as m from '$lib/paraglide/messages.js';
+import { timelineDateLabel } from './dateLabel';
+import type { components } from '$lib/generated/api';
+
+type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
+
+/**
+ * A single archive letter on the timeline: sender → receiver, title, and a
+ * precision-aware date chip, linking to the document. Names/titles are
+ * OCR/import-derived — rendered via default `{...}` escaping with
+ * `whitespace-pre-line` for line breaks (REQ-021); never `{@html}`.
+ */
+let { entry }: { entry: TimelineEntryDTO } = $props();
+
+const dateLabel = $derived(timelineDateLabel(entry.eventDate, entry.precision, entry.eventDateEnd));
+const sender = $derived(entry.senderName === '' ? m.timeline_unknown_person() : entry.senderName);
+const receiver = $derived(
+	entry.receiverName === '' ? m.timeline_unknown_person() : entry.receiverName
+);
+</script>
+
+<!-- Box layout inline (not just utility classes) so the 44px touch target holds
+     even before the stylesheet loads — an <a> is inline by default and would
+     ignore min-height otherwise. WCAG 2.5.5 (REQ-020). -->
+<a
+	href="/documents/{entry.documentId}"
+	style="display: flex; flex-direction: column; justify-content: center; min-height: 44px"
+	class="rounded-sm border border-l-[3px] border-line border-l-brand-mint bg-surface px-3 py-2 shadow-sm transition-colors hover:border-brand-mint focus:outline-none focus-visible:ring-2 focus-visible:ring-brand-navy"
+>
+	{#if entry.title}
+		<span class="font-serif text-sm font-bold whitespace-pre-line text-ink">{entry.title}</span>
+	{/if}
+	<span class="mt-0.5 font-sans text-xs text-ink-3">
+		<span class="font-serif whitespace-pre-line">{sender}</span>
+		<span aria-hidden="true">→</span>
+		<span class="font-serif whitespace-pre-line">{receiver}</span>
+		{#if dateLabel}
+			<span data-testid="letter-date"> · {dateLabel}</span>
+		{/if}
+	</span>
+</a>

frontend/src/lib/timeline/LetterCard.svelte.spec.ts

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+import { describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import LetterCard from './LetterCard.svelte';
+import { timelineDateLabel } from './dateLabel';
+import { makeEntry } from './test-factories';
+
+afterEach(() => cleanup());
+
+const DOC_ID = '22222222-2222-2222-2222-222222222222';
+
+describe('LetterCard', () => {
+	it('renders sender, receiver, and title', () => {
+		render(LetterCard, {
+			entry: makeEntry({ senderName: 'Karl', receiverName: 'Elfriede', title: 'Feldpost' })
+		});
+		expect(document.body.textContent).toContain('Karl');
+		expect(document.body.textContent).toContain('Elfriede');
+		expect(document.body.textContent).toContain('Feldpost');
+	});
+
+	it('renders the precision date exactly as timelineDateLabel returns (REQ-013)', () => {
+		const entry = makeEntry({ eventDate: '1915-06-15', precision: 'MONTH' });
+		const expected = timelineDateLabel(entry.eventDate, entry.precision, entry.eventDateEnd);
+		expect(expected).toBeTruthy();
+		render(LetterCard, { entry });
+		expect(document.body.textContent).toContain(expected as string);
+	});
+
+	it('renders no date chip when timelineDateLabel returns null (REQ-013)', () => {
+		const entry = makeEntry({ precision: 'UNKNOWN', eventDate: undefined });
+		render(LetterCard, { entry });
+		const chip = document.querySelector('[data-testid="letter-date"]');
+		expect(chip).toBeNull();
+	});
+
+	it('shows "Unbekannt" for an empty sender, never a bare arrow (REQ-014)', () => {
+		render(LetterCard, { entry: makeEntry({ senderName: '', receiverName: 'Elfriede' }) });
+		expect(document.body.textContent).toContain('Unbekannt');
+	});
+
+	it('shows "Unbekannt" for an empty receiver (REQ-014)', () => {
+		render(LetterCard, { entry: makeEntry({ senderName: 'Karl', receiverName: '' }) });
+		expect(document.body.textContent).toContain('Unbekannt');
+	});
+
+	it('links to exactly /documents/{documentId} with no target (REQ-023)', () => {
+		render(LetterCard, { entry: makeEntry({ documentId: DOC_ID }) });
+		const link = document.querySelector('a') as HTMLAnchorElement;
+		expect(link.getAttribute('href')).toBe(`/documents/${DOC_ID}`);
+		expect(link.hasAttribute('target')).toBe(false);
+	});
+
+	it('has a touch target of at least 44px (REQ-020)', () => {
+		render(LetterCard, { entry: makeEntry() });
+		const link = document.querySelector('a') as HTMLAnchorElement;
+		expect(link.getBoundingClientRect().height).toBeGreaterThanOrEqual(44);
+	});
+});