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/src/lib/document/TimelineBars.svelte

@@ -51,10 +51,12 @@ function barHeight(count: number): number {
 		<button
 			type="button"
 			data-testid="timeline-bar"
-			aria-label={m.timeline_bar_aria({
-				when: formatTickLabel(bucket.month, getLocale()),
-				count: bucket.count
-			})}
+			aria-label={bucket.count === 1
+				? m.timeline_bar_aria_singular({ when: formatTickLabel(bucket.month, getLocale()) })
+				: m.timeline_bar_aria_plural({
+						when: formatTickLabel(bucket.month, getLocale()),
+						count: bucket.count
+					})}
 			aria-pressed={isSelected(bucket.month)}
 			onpointerdown={(e) => onbarpointerdown(e, i)}
 			onpointerenter={() => onbarpointerenter(i)}