feat(timeline): add a compact LetterCard variant for in-bucket letters

Grouped-mode buckets stack many letters; the full two-line card with its own date
chip floods the view. The compact variant tightens the padding and, when the
letter has a title, drops the redundant date chip (the per-year bucket already
frames the time and these archive titles embed the date). Datum mode is untouched
— compact defaults to false.

Refs #827

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

@@ -151,4 +151,22 @@ describe('LetterCard — grouping variants (#827, REQ-014/017)', () => {
 		render(LetterCard, { entry: makeEntry({ rootTagName: 'Krieg', rootTagColor: 'sienna' }) });
 		expect(document.querySelector('[data-testid="tag-chip"]')).not.toBeNull();
 	});
+
+	it('drops the redundant date line in the compact variant when a title is present (#827)', () => {
+		// Inside a per-year bucket the year already frames the time, and these archive
+		// titles embed the date — so the compact in-bucket card omits the date chip.
+		render(LetterCard, { entry: makeEntry({ title: 'H-0023 – 6. Juli 1916' }), compact: true });
+		expect(document.querySelector('[data-testid="letter-date"]')).toBeNull();
+		expect(document.body.textContent).toContain('Karl Raddatz'); // sender still shown
+	});
+
+	it('keeps the date in the compact variant when the letter has no title (#827)', () => {
+		render(LetterCard, { entry: makeEntry({ title: undefined }), compact: true });
+		expect(document.querySelector('[data-testid="letter-date"]')).not.toBeNull();
+	});
+
+	it('renders the compact variant on a single tighter row (#827)', () => {
+		render(LetterCard, { entry: makeEntry(), compact: true });
+		expect(document.querySelector('a.lcard.compact')).not.toBeNull();
+	});
 });