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

Eight Paraglide keys for the /zeitstrahl layer filter — layer labels, the
fieldset legend, the sticky trigger (distinct timeline_filter_trigger and
timeline_filter_trigger_active({count}) so it never reads "Filter (0 aktiv)"),
the reset button, and the filtered-empty message — added to all three locales.
messages.spec asserts their presence and the {count} signature.

Refs #780
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/messages/es.json

@@ -1057,6 +1057,14 @@
 	"timeline_events_count": "{count} eventos",
 	"timeline_letters_count_singular": "1 carta",
 	"timeline_events_count_singular": "1 evento",
+	"timeline_filter_label_layers": "Mostrar capas",
+	"timeline_filter_layer_personal": "Eventos personales",
+	"timeline_filter_layer_historical": "Eventos históricos",
+	"timeline_filter_layer_letters": "Cartas",
+	"timeline_filter_trigger": "Filtro",
+	"timeline_filter_trigger_active": "Filtro ({count} activos)",
+	"timeline_filter_reset": "Restablecer filtros",
+	"timeline_filter_empty_state": "Ninguna entrada coincide con estos filtros.",
 	"event_editor_new_title": "Nuevo evento",
 	"event_editor_edit_title": "Editar evento",
 	"event_editor_section_when": "Cuándo",