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/utils/exchange.ts

@@ -7,7 +7,7 @@
  * e.g., 10000 -> "€100"
  */
 export function formatEur(cents: number): string {
-  return `€${(cents / 100).toLocaleString("de-DE")}`;
+  return `€${(cents / 100).toLocaleString("es-ES")}`;
 }
 
 /**