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/BookingStep.tsx

@@ -277,10 +277,10 @@ export function BookingStep({
                 }}
               >
                 <div style={styles.dateWeekday}>
-                  {date.toLocaleDateString("en-US", { weekday: "short" })}
+                  {date.toLocaleDateString("es-ES", { weekday: "short" })}
                 </div>
                 <div style={styles.dateDay}>
-                  {date.toLocaleDateString("en-US", {
+                  {date.toLocaleDateString("es-ES", {
                     month: "short",
                     day: "numeric",
                   })}
@@ -312,7 +312,7 @@ export function BookingStep({
         <div style={styles.section}>
           <h2 style={styles.sectionTitle}>
             {t("bookingStep.availableSlots")}{" "}
-            {selectedDate.toLocaleDateString("en-US", {
+            {selectedDate.toLocaleDateString("es-ES", {
               weekday: "long",
               month: "long",
               day: "numeric",