fix: use static month names in getTimeRange to prevent OS locale leaking into sidebar (#22454)

getTimeRange returns month names that are used as i18n translation keys
(consumed via \.t(chat.time_range) in the sidebar, search modal, etc.).
The keys must be exact English strings like 'January', 'February', etc.

Previously, toLocaleString('default', { month: 'long' }) was used to
generate these keys. The 'default' locale defers to the browser's locale
resolution, which in Firefox with intl.regional_prefs.use_os_locales=true
picks up OS regional settings instead of the browser language. This caused
German month names (e.g. 'Februar', 'Januar') to appear in the sidebar for
users whose OS region is set to Germany, even when both browser and app
language are set to English. Chrome was unaffected because it ignores OS
regional settings for the 'default' locale.

Since i18n has no translation key for 'Februar', the German string passed
through untranslated. Replace toLocaleString with a static MONTH_NAMES
array lookup to make the intent explicit and eliminate any browser/OS
locale dependency.

src/lib/utils/index.ts

@@ -1094,6 +1094,22 @@ export const approximateToHumanReadable = (nanoseconds: number) => {
 	return results.reverse().join(' ');
 };
 
+// Month names used as i18n translation keys — must be English regardless of locale
+const MONTH_NAMES = [
+	'January',
+	'February',
+	'March',
+	'April',
+	'May',
+	'June',
+	'July',
+	'August',
+	'September',
+	'October',
+	'November',
+	'December'
+];
+
 export const getTimeRange = (timestamp) => {
 	const now = new Date();
 	const date = new Date(timestamp * 1000); // Convert Unix timestamp to milliseconds
@@ -1119,7 +1135,7 @@ export const getTimeRange = (timestamp) => {
 	} else if (diffDays <= 30) {
 		return 'Previous 30 days';
 	} else if (nowYear === dateYear) {
-		return date.toLocaleString('default', { month: 'long' });
+		return MONTH_NAMES[dateMonth];
 	} else {
 		return date.getFullYear().toString();
 	}