feat(timeline): show the root-tag chip on the letter card

LetterCard now renders a TagChip beneath the sender→receiver/date line
whenever the entry carries a rootTagName, mapping rootTagColor to the chip
(neutral when null). Because the chip lives on LetterCard it shows up wherever
a LetterCard does — the global timeline and the expanded YearLetterStrip — with
no per-surface special-casing; a tagless letter shows no chip. A long name
truncates inline so the card never overflows at 320px.

Refs #835
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -1,7 +1,9 @@
 import { describe, it, expect, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
+import { tick } from 'svelte';
 import * as m from '$lib/paraglide/messages.js';
 import LetterCard from './LetterCard.svelte';
+import YearLetterStrip from './YearLetterStrip.svelte';
 import { timelineDateLabel } from './dateLabel';
 import { makeEntry } from './test-factories';
 
@@ -86,4 +88,42 @@ describe('LetterCard', () => {
 		expect(document.body.textContent).toContain(evil);
 		expect(document.querySelector('a script')).toBeNull();
 	});
+
+	it('renders one root-tag chip beneath the meta line when rootTagName is present (REQ-008)', () => {
+		render(LetterCard, { entry: makeEntry({ rootTagName: 'Familie', rootTagColor: 'sage' }) });
+		const chips = document.querySelectorAll('[data-testid="tag-chip"]');
+		expect(chips).toHaveLength(1);
+		expect(chips[0].textContent).toContain('Familie');
+	});
+
+	it('renders no chip when the letter has no root tag (REQ-005/006)', () => {
+		render(LetterCard, { entry: makeEntry({ rootTagName: undefined, rootTagColor: undefined }) });
+		expect(document.querySelector('[data-testid="tag-chip"]')).toBeNull();
+	});
+
+	it('keeps a long tag name from overflowing the card at 320px, full name in the title (REQ-008a)', () => {
+		document.body.style.width = '320px';
+		render(LetterCard, {
+			entry: makeEntry({
+				rootTagName: 'Briefe von der Front und aus der Heimat',
+				rootTagColor: 'sienna'
+			})
+		});
+		const link = document.querySelector('a') as HTMLAnchorElement;
+		expect(link.scrollWidth).toBeLessThanOrEqual(link.clientWidth);
+		const chip = document.querySelector('[data-testid="tag-chip"]') as HTMLElement;
+		expect(chip.getAttribute('title')).toBe('Briefe von der Front und aus der Heimat');
+		document.body.style.width = '';
+	});
+
+	it('renders the chip inside an expanded YearLetterStrip too (REQ-012)', async () => {
+		render(YearLetterStrip, {
+			letters: [makeEntry({ rootTagName: 'Familie', rootTagColor: 'sage', documentId: 'doc-1' })],
+			year: 1909
+		});
+		(document.querySelector('[data-testid="strip-expand"]') as HTMLButtonElement).click();
+		await tick();
+		const chip = document.querySelector('[data-testid="tag-chip"]');
+		expect(chip?.textContent).toContain('Familie');
+	});
 });