feat(timeline): add the EventCluster card + show-more/less i18n

A curated event with linked letters renders as one contained card: the
event is the header (accent glyph, title, date · provenance, count, and a
curator edit link), its letters sit inside as compact .lcard.ev cards. The
first CLUSTER_PREVIEW (5) show, then a keyboard-operable show-more/less
toggle reveals the rest. A cross-year card (no event prop) gets a plain
'✉ title' text header with no edit link. Titles render through default
{...} escaping. Adds timeline_bucket_show_more/less keys to de/en/es.

Refs #850

frontend/messages/de.json

@@ -1052,6 +1052,8 @@
 	"timeline_grouping_date": "Gruppierung: Datum",
 	"timeline_provenance_derived": "abgeleitet",
 	"timeline_provenance_curated": "kuratiert",
+	"timeline_bucket_show_more": "+ {count} weitere Briefe anzeigen",
+	"timeline_bucket_show_less": "Weniger anzeigen",
 	"timeline_letter_glyph_label": "Brief",
 	"timeline_tag_chip_label": "Thema",
 	"timeline_layer_historical_suffix": "historisch",

frontend/messages/en.json

@@ -1052,6 +1052,8 @@
 	"timeline_grouping_date": "Grouping: Date",
 	"timeline_provenance_derived": "derived",
 	"timeline_provenance_curated": "curated",
+	"timeline_bucket_show_more": "+ {count} more letters",
+	"timeline_bucket_show_less": "Show fewer",
 	"timeline_letter_glyph_label": "Letter",
 	"timeline_tag_chip_label": "Topic",
 	"timeline_layer_historical_suffix": "historical",

frontend/messages/es.json

@@ -1052,6 +1052,8 @@
 	"timeline_grouping_date": "Agrupación: Fecha",
 	"timeline_provenance_derived": "derivado",
 	"timeline_provenance_curated": "curado",
+	"timeline_bucket_show_more": "+ {count} cartas más",
+	"timeline_bucket_show_less": "Mostrar menos",
 	"timeline_letter_glyph_label": "Carta",
 	"timeline_tag_chip_label": "Tema",
 	"timeline_layer_historical_suffix": "histórico",