feat(i18n): add timeline density widget keys (#385)

Five new keys across de/en/es for the upcoming TimelineDensityFilter:
aria label, clear selection, abbreviated count label, loading state, and
parametrised filtered-count message.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -1045,5 +1045,11 @@
 	"relation_form_year_placeholder": "z.B. 1920",
 
 	"person_relationships_heading": "Beziehungen",
-	"person_relationships_empty": "Noch keine Beziehungen bekannt."
+	"person_relationships_empty": "Noch keine Beziehungen bekannt.",
+
+	"timeline_aria_label": "Zeitachse Dokumentdichte",
+	"timeline_clear_selection": "Auswahl zurücksetzen",
+	"timeline_count_label": "Dok.",
+	"timeline_loading": "Lade Zeitachse…",
+	"timeline_filtered_count": "{count} Dokumente im Zeitraum"
 }

frontend/messages/en.json

@@ -1045,5 +1045,11 @@
 	"relation_form_year_placeholder": "e.g. 1920",
 
 	"person_relationships_heading": "Relationships",
-	"person_relationships_empty": "No relationships known yet."
+	"person_relationships_empty": "No relationships known yet.",
+
+	"timeline_aria_label": "Document density timeline",
+	"timeline_clear_selection": "Clear selection",
+	"timeline_count_label": "docs",
+	"timeline_loading": "Loading timeline…",
+	"timeline_filtered_count": "{count} documents in range"
 }

frontend/messages/es.json

@@ -1045,5 +1045,11 @@
 	"relation_form_year_placeholder": "ej. 1920",
 
 	"person_relationships_heading": "Relaciones",
-	"person_relationships_empty": "Aún no se conocen relaciones."
+	"person_relationships_empty": "Aún no se conocen relaciones.",
+
+	"timeline_aria_label": "Cronología de densidad de documentos",
+	"timeline_clear_selection": "Borrar selección",
+	"timeline_count_label": "docs",
+	"timeline_loading": "Cargando cronología…",
+	"timeline_filtered_count": "{count} documentos en el rango"
 }