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/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);