feat(documents): localise timeline bar aria-label (#385)

Replaces the raw "1915-08 · 5" aria-label, which a screen reader
announces as "1915 dash 08 middle dot 5", with the i18n template
timeline_bar_aria("{when}, {count} ...") and a getLocale-formatted
month/year string. Closes Leonie's WCAG 1.3.1 / 4.1.2 finding and
Felix's localisation flag.

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

frontend/messages/en.json

@@ -1050,5 +1050,6 @@
 	"timeline_aria_label": "Document density timeline",
 	"timeline_clear_selection": "Clear selection",
 	"timeline_zoom_reset": "Reset zoom",
+	"timeline_bar_aria": "{when}, {count} documents",
 	"timeline_dragging_aria_live": "Range {from} to {to} selected"
 }