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/exchange/components/ConfirmationStep.tsx

@@ -30,7 +30,7 @@ interface ConfirmationStepProps {
  * Format price for display
  */
 function formatPrice(price: number): string {
-  return `€${price.toLocaleString("de-DE", { maximumFractionDigits: 0 })}`;
+  return `€${price.toLocaleString("es-ES", { maximumFractionDigits: 0 })}`;
 }
 
 const styles: Record<string, CSSProperties> = {
@@ -168,7 +168,7 @@ export function ConfirmationStep({
         </div>
         <div style={styles.compressedBookingDetails}>
           <span>
-            {selectedDate?.toLocaleDateString("en-US", {
+            {selectedDate?.toLocaleDateString("es-ES", {
               weekday: "short",
               month: "short",
               day: "numeric",