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/PriceDisplay.tsx

@@ -19,7 +19,7 @@ interface PriceDisplayProps {
  * 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> = {
@@ -121,7 +121,7 @@ export function PriceDisplay({
           </div>
           {lastUpdate && (
             <div style={styles.priceTimestamp}>
-              {t("priceDisplay.updated")} {lastUpdate.toLocaleTimeString()}
+              {t("priceDisplay.updated")} {lastUpdate.toLocaleTimeString("es-ES")}
               {isPriceStale && <span style={styles.staleWarning}> {t("priceDisplay.stale")}</span>}
             </div>
           )}