fix(documents): pluralise timeline bar aria-label by count (#385)

The flat "{count} Dokumente / documents / documentos" keys read as
"1 Dokumente" / "1 documents" / "1 documentos" to a screen reader
when only one document falls in the month bucket. Splits each
locale into _singular + _plural keys and picks the form by count
in TimelineBars, mirroring the existing upload_banner_singular /
_plural pattern in this project.

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

frontend/messages/de.json

@@ -1050,6 +1050,7 @@
 	"timeline_aria_label": "Zeitachse Dokumentdichte",
 	"timeline_clear_selection": "Auswahl zurücksetzen",
 	"timeline_zoom_reset": "Zurück zur Übersicht",
-	"timeline_bar_aria": "{when}, {count} Dokumente",
+	"timeline_bar_aria_singular": "{when}, 1 Dokument",
+	"timeline_bar_aria_plural": "{when}, {count} Dokumente",
 	"timeline_dragging_aria_live": "Zeitraum {from} bis {to} ausgewählt"
 }

frontend/messages/en.json

@@ -1050,6 +1050,7 @@
 	"timeline_aria_label": "Document density timeline",
 	"timeline_clear_selection": "Clear selection",
 	"timeline_zoom_reset": "Reset zoom",
-	"timeline_bar_aria": "{when}, {count} documents",
+	"timeline_bar_aria_singular": "{when}, 1 document",
+	"timeline_bar_aria_plural": "{when}, {count} documents",
 	"timeline_dragging_aria_live": "Range {from} to {to} selected"
 }

frontend/messages/es.json

@@ -1050,6 +1050,7 @@
 	"timeline_aria_label": "Cronología de densidad de documentos",
 	"timeline_clear_selection": "Borrar selección",
 	"timeline_zoom_reset": "Restablecer zoom",
-	"timeline_bar_aria": "{when}, {count} documentos",
+	"timeline_bar_aria_singular": "{when}, 1 documento",
+	"timeline_bar_aria_plural": "{when}, {count} documentos",
 	"timeline_dragging_aria_live": "Rango {from} a {to} seleccionado"
 }