fix(documents): collapse timeline to year bars when range > 240 months (#385)

Surfaced during proofshot: the production archive spans 1873 → 2023
(≈1809 month bars). With flex-1 + gap-px on a 1280 px container, every
pixel was consumed by gaps and bars rendered at 0 px width — visible as
"empty box, no bars".

Fix:
- Add aggregateToYears(buckets) that sums month counts per year and
  returns YYYY-keyed entries.
- Add selectionBoundaryFrom/To that handle both YYYY and YYYY-MM labels
  (Jan 1 → Dec 31 for years, first → last day for months).
- Component switches to year granularity when the gap-filled month
  sequence exceeds 240 entries (~20 years), keeping each bar clickable.
- Drop the gap-px between bars and add min-w-px so sub-pixel rounding
  still leaves something visible.

5 new tests cover aggregation, boundary helpers, and the component-level
year-mode + click behaviour.

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

frontend/src/lib/document/timeline.spec.ts

@@ -4,7 +4,10 @@ import {
 	monthBoundaryTo,
 	buildMonthSequence,
 	fillDensityGaps,
-	fetchDensity
+	fetchDensity,
+	aggregateToYears,
+	selectionBoundaryFrom,
+	selectionBoundaryTo
 } from './timeline';
 
 describe('monthBoundaryFrom', () => {
@@ -108,6 +111,47 @@ describe('fillDensityGaps', () => {
 	});
 });
 
+describe('aggregateToYears', () => {
+	it('returns empty array for empty input', () => {
+		expect(aggregateToYears([])).toEqual([]);
+	});
+
+	it('sums counts within the same year', () => {
+		const result = aggregateToYears([
+			{ month: '1915-08', count: 5 },
+			{ month: '1915-09', count: 2 },
+			{ month: '1915-10', count: 8 }
+		]);
+		expect(result).toEqual([{ month: '1915', count: 15 }]);
+	});
+
+	it('produces one bucket per distinct year, sorted chronologically', () => {
+		const result = aggregateToYears([
+			{ month: '1916-01', count: 3 },
+			{ month: '1915-08', count: 5 },
+			{ month: '1916-04', count: 7 },
+			{ month: '1914-12', count: 1 }
+		]);
+		expect(result).toEqual([
+			{ month: '1914', count: 1 },
+			{ month: '1915', count: 5 },
+			{ month: '1916', count: 10 }
+		]);
+	});
+});
+
+describe('selectionBoundaryFrom / To', () => {
+	it('handles month labels (YYYY-MM)', () => {
+		expect(selectionBoundaryFrom('1915-08')).toBe('1915-08-01');
+		expect(selectionBoundaryTo('1915-08')).toBe('1915-08-31');
+	});
+
+	it('handles year labels (YYYY)', () => {
+		expect(selectionBoundaryFrom('1915')).toBe('1915-01-01');
+		expect(selectionBoundaryTo('1915')).toBe('1915-12-31');
+	});
+});
+
 describe('fetchDensity', () => {
 	it('skips fetch and returns null density on mobile', async () => {
 		const fetch = vi.fn();