feat(timeline): add YearLetterStrip for dense years

Letter count + 12-month density sparkline + a >=44px keyboard-focusable expand
toggle that reveals that year's LetterCards (REQ-012). Sparkline values from the
shared monthHistogram.

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

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

@@ -0,0 +1,42 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { tick } from 'svelte';
+import YearLetterStrip from './YearLetterStrip.svelte';
+import { makeEntry } from './test-factories';
+
+afterEach(() => cleanup());
+
+function denseLetters(year: number, count: number) {
+	return Array.from({ length: count }, (_, i) =>
+		makeEntry({
+			eventDate: `${year}-${String((i % 12) + 1).padStart(2, '0')}-10`,
+			documentId: `doc-${i}`
+		})
+	);
+}
+
+describe('YearLetterStrip', () => {
+	it('shows the letter count and a 12-bar sparkline (REQ-012)', () => {
+		render(YearLetterStrip, { letters: denseLetters(1915, 30), year: 1915 });
+		expect(document.body.textContent).toContain('30');
+		const bars = document.querySelectorAll('[data-testid="sparkline-bar"]');
+		expect(bars).toHaveLength(12);
+	});
+
+	it('has a keyboard-focusable expand toggle of at least 44px (REQ-012)', () => {
+		render(YearLetterStrip, { letters: denseLetters(1915, 30), year: 1915 });
+		const toggle = document.querySelector('[data-testid="strip-expand"]') as HTMLButtonElement;
+		expect(toggle).not.toBeNull();
+		expect(toggle.tagName).toBe('BUTTON');
+		expect(toggle.getBoundingClientRect().height).toBeGreaterThanOrEqual(44);
+	});
+
+	it('reveals all letter cards when expanded (REQ-012)', async () => {
+		render(YearLetterStrip, { letters: denseLetters(1915, 30), year: 1915 });
+		expect(document.querySelectorAll('a').length).toBe(0);
+		const toggle = document.querySelector('[data-testid="strip-expand"]') as HTMLButtonElement;
+		toggle.click();
+		await tick();
+		expect(document.querySelectorAll('a').length).toBe(30);
+	});
+});