Fix date/time formatting to use es-ES locale

- Update all date/time formatting functions to use 'es-ES' locale instead of 'en-US' or 'de-DE'
- Update utility functions in utils/date.ts and utils/exchange.ts
- Update all component files that use date formatting
- Update e2e test helper to match new Spanish date format
- All formatting now uses Spanish locale regardless of selected language as per PR requirements

frontend/app/admin/invites/page.tsx

@@ -116,7 +116,7 @@ export default function AdminInvitesPage() {
   };
 
   const formatDate = (dateStr: string) => {
-    return new Date(dateStr).toLocaleString();
+    return new Date(dateStr).toLocaleString("es-ES");
   };
 
   const getStatusBadgeVariant = (status: string): "ready" | "success" | "error" | undefined => {