feat(timeline): add timeline i18n keys (de/en/es)

14 Paraglide keys for the /zeitstrahl view: nav link, heading, empty/undated/
gap/unknown-person chrome, letters count, strip expand, range aria, and the
layer/derived labels. The layer (Weltgeschehen/Familie) and derived (Geburt/
Tod/Heirat) labels carry the German term across all locales by design
(documented MVP decision). REQ-024.

Refs #779
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/messages/es.json

@@ -1032,6 +1032,20 @@
 	"bulk_edit_count_pill": "Se editarán {count}",
 	"nav_stammbaum": "Árbol genealógico",
 	"nav_geschichten": "Historias",
+	"nav_zeitstrahl": "Línea de tiempo",
+	"timeline_heading": "Línea de tiempo",
+	"timeline_empty_state": "Aún no hay eventos.",
+	"timeline_undated_section": "Sin Fecha",
+	"timeline_unknown_person": "Desconocido",
+	"timeline_gap_empty": "sin entradas",
+	"timeline_letters_count": "{count} cartas",
+	"timeline_strip_expand": "Mostrar cartas",
+	"timeline_range_aria": "Período: {from} a {to}",
+	"timeline_layer_world": "Weltgeschehen",
+	"timeline_layer_family": "Familie",
+	"timeline_derived_birth": "Geburt",
+	"timeline_derived_death": "Tod",
+	"timeline_derived_marriage": "Heirat",
 	"error_geschichte_not_found": "No se encontró la historia.",
 	"error_journey_item_not_found": "No se encontró el elemento del viaje.",
 	"error_journey_item_position_conflict": "El orden fue cambiado por otra persona — por favor recargue la página.",