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/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",