chore(i18n): remove dead timeline keys (#385)

The timeline_count_label, timeline_loading, timeline_filtered_count,
and timeline_zoom_in keys were never referenced from src/. Felix's
review flagged them as 15 dead strings to translate. Removed across
de/en/es; the timeline_dragging_aria_live key is kept and will be
wired up in the next commit.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/messages/de.json

@@ -1049,10 +1049,6 @@
 
 	"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",
-	"timeline_zoom_in": "Zeitraum vergrößern",
 	"timeline_zoom_reset": "Zurück zur Übersicht",
 	"timeline_dragging_aria_live": "Zeitraum {from} bis {to} ausgewählt"
 }

frontend/messages/en.json

@@ -1049,10 +1049,6 @@
 
 	"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",
-	"timeline_zoom_in": "Zoom in",
 	"timeline_zoom_reset": "Reset zoom",
 	"timeline_dragging_aria_live": "Range {from} to {to} selected"
 }

frontend/messages/es.json

@@ -1049,10 +1049,6 @@
 
 	"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",
-	"timeline_zoom_in": "Acercar",
 	"timeline_zoom_reset": "Restablecer zoom",
 	"timeline_dragging_aria_live": "Rango {from} a {to} seleccionado"
 }