fix(timeline): pluralize the zeitstrahl meta-line counts

A count of one rendered "1 Briefe" / "1 Ereignisse". Add _singular
companion keys (de/en/es) and select them when the count is exactly one,
following the project's _singular/_plural convention.

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

frontend/messages/de.json

@@ -1053,6 +1053,8 @@
 	"timeline_layer_historical_suffix": "historisch",
 	"timeline_strip_density_caption": "Monats-Dichte",
 	"timeline_events_count": "{count} Ereignisse",
+	"timeline_letters_count_singular": "1 Brief",
+	"timeline_events_count_singular": "1 Ereignis",
 	"event_editor_new_title": "Neues Ereignis",
 	"event_editor_edit_title": "Ereignis bearbeiten",
 	"event_editor_section_when": "Wann",

frontend/messages/en.json

@@ -1053,6 +1053,8 @@
 	"timeline_layer_historical_suffix": "historical",
 	"timeline_strip_density_caption": "Monthly density",
 	"timeline_events_count": "{count} events",
+	"timeline_letters_count_singular": "1 letter",
+	"timeline_events_count_singular": "1 event",
 	"event_editor_new_title": "New event",
 	"event_editor_edit_title": "Edit event",
 	"event_editor_section_when": "When",

frontend/messages/es.json

@@ -1053,6 +1053,8 @@
 	"timeline_layer_historical_suffix": "histórico",
 	"timeline_strip_density_caption": "Densidad mensual",
 	"timeline_events_count": "{count} eventos",
+	"timeline_letters_count_singular": "1 carta",
+	"timeline_events_count_singular": "1 evento",
 	"event_editor_new_title": "Nuevo evento",
 	"event_editor_edit_title": "Editar evento",
 	"event_editor_section_when": "Cuándo",

frontend/src/lib/messages.spec.ts

@@ -80,7 +80,9 @@ describe('message key parity', () => {
 			'timeline_letter_glyph_label',
 			'timeline_layer_historical_suffix',
 			'timeline_strip_density_caption',
-			'timeline_events_count'
+			'timeline_events_count',
+			'timeline_letters_count_singular',
+			'timeline_events_count_singular'
 		];
 		for (const key of requiredKeys) {
 			expect(de, `missing key in de: ${key}`).toHaveProperty(key);

frontend/src/routes/zeitstrahl/+page.svelte

@@ -18,12 +18,21 @@ const metaLine = $derived.by(() => {
 	if (meta.firstYear !== null && meta.lastYear !== null) {
 		segments.push(`${meta.firstYear}–${meta.lastYear}`);
 	}
-	// A zero-count segment ("0 Briefe") reads as a data error — drop it.
+	// A zero-count segment ("0 Briefe") reads as a data error — drop it; a count
+	// of one takes the singular key ("1 Brief"), per the project plural convention.
 	if (meta.letterCount > 0) {
-		segments.push(m.timeline_letters_count({ count: meta.letterCount }));
+		segments.push(
+			meta.letterCount === 1
+				? m.timeline_letters_count_singular()
+				: m.timeline_letters_count({ count: meta.letterCount })
+		);
 	}
 	if (meta.eventCount > 0) {
-		segments.push(m.timeline_events_count({ count: meta.eventCount }));
+		segments.push(
+			meta.eventCount === 1
+				? m.timeline_events_count_singular()
+				: m.timeline_events_count({ count: meta.eventCount })
+		);
 	}
 	segments.push(m.timeline_grouping_date());
 	return segments.join(' · ');

frontend/src/routes/zeitstrahl/page.svelte.spec.ts

@@ -68,7 +68,7 @@ describe('/zeitstrahl page', () => {
 		const sub = document.querySelector('[data-testid="timeline-meta"]');
 		expect(sub).not.toBeNull();
 		expect(sub?.textContent).not.toContain('–');
-		expect(sub?.textContent).toContain(m.timeline_letters_count({ count: 1 }));
+		expect(sub?.textContent).toContain(m.timeline_letters_count_singular());
 	});
 
 	it('omits the entire sub-line for an empty timeline (REQ-002)', () => {
@@ -94,4 +94,20 @@ describe('/zeitstrahl page', () => {
 		expect(sub).not.toBeNull();
 		expect(sub?.textContent).not.toContain(m.timeline_events_count({ count: 0 }));
 	});
+
+	it('uses singular count labels for exactly one letter and one event (REQ-002)', () => {
+		render(Page, {
+			data: pageData(
+				makeTimelineDTO({
+					years: [makeYear(1914, [makeEntry({ documentId: 'a' }), event('Geburt')])]
+				})
+			)
+		});
+		const sub = document.querySelector('[data-testid="timeline-meta"]');
+		expect(sub?.textContent).toContain(m.timeline_letters_count_singular());
+		expect(sub?.textContent).toContain(m.timeline_events_count_singular());
+		// never the "1 Briefe"/"1 Ereignisse" plural forms for a count of one
+		expect(sub?.textContent).not.toContain(m.timeline_letters_count({ count: 1 }));
+		expect(sub?.textContent).not.toContain(m.timeline_events_count({ count: 1 }));
+	});
 });