feat(timeline): render letter buckets in TimelineView/YearBand

Thread groupingMode through TimelineView → YearBand. TimelineView resolves the
event lookup once over the filtered view (so Ereignis clusters never reference a
filtered-out event). In non-Datum modes YearBand keeps its event pills/world-bands
identical (REQ-001) and replaces the loose letters with per-year LetterBuckets
(REQ-002/003/004); Datum keeps the original card/strip path. The undated bucket is
unchanged in every mode.

Refs #827
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -165,3 +165,60 @@ describe('YearBand', () => {
 		}
 	});
 });
+
+describe('YearBand — grouping modes (#827)', () => {
+	it('keeps individual letter cards and no buckets in Datum mode (default)', () => {
+		render(YearBand, { year: makeYear(1915, manyLetters(1915, 3)) });
+		expect(document.querySelector('[data-testid="letter-bucket"]')).toBeNull();
+		expect(document.querySelectorAll('a')).toHaveLength(3);
+	});
+
+	it('clusters loose letters under their linked event in Ereignis mode (REQ-002/003)', () => {
+		const a = makeEntry({ documentId: 'a', linkedEventId: 'e1', eventDate: '1915-03-01' });
+		const b = makeEntry({ documentId: 'b', linkedEventId: 'e1', eventDate: '1915-04-01' });
+		render(YearBand, {
+			year: makeYear(1915, [a, b]),
+			groupingMode: 'event',
+			eventLookup: new Map([['e1', 'Briefe von der Front']])
+		});
+		expect(document.querySelectorAll('[data-testid="letter-bucket"]')).toHaveLength(1);
+		expect(document.body.textContent).toContain('Briefe von der Front');
+		// no alternating individual letter rows in grouped mode
+		expect(document.querySelector('.letter-row')).toBeNull();
+	});
+
+	it('still renders the event world-band in Ereignis mode (REQ-001)', () => {
+		const band = makeEntry({
+			kind: 'EVENT',
+			type: 'HISTORICAL',
+			precision: 'RANGE',
+			eventDate: '1914-01-01',
+			eventDateEnd: '1918-12-31',
+			title: 'Erster Weltkrieg',
+			senderName: '',
+			receiverName: '',
+			documentId: undefined
+		});
+		const letter = makeEntry({ documentId: 'a', linkedEventId: 'e1', eventDate: '1914-05-01' });
+		render(YearBand, {
+			year: makeYear(1914, [band, letter]),
+			groupingMode: 'event',
+			eventLookup: new Map([['e1', 'Front']])
+		});
+		expect(document.querySelector('[data-testid="world-range"]')).not.toBeNull();
+		expect(document.querySelector('[data-testid="letter-bucket"]')).not.toBeNull();
+	});
+
+	it('buckets loose letters under their root tag in Thema mode (REQ-004)', () => {
+		const a = makeEntry({
+			documentId: 'a',
+			rootTagId: 't1',
+			rootTagName: 'Krieg',
+			rootTagColor: 'sienna',
+			eventDate: '1915-03-01'
+		});
+		render(YearBand, { year: makeYear(1915, [a]), groupingMode: 'thema', eventLookup: new Map() });
+		const chip = document.querySelector('[data-testid="bucket-header-chip"]');
+		expect(chip?.textContent).toContain('Krieg');
+	});
+});