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/trades/[id]/page.tsx

@@ -154,7 +154,7 @@ export default function TradeDetailPage() {
                 <span style={styles.detailLabel}>Market Price:</span>
                 <span style={styles.detailValue}>
                   €
-                  {trade.market_price_eur.toLocaleString("de-DE", {
+                  {trade.market_price_eur.toLocaleString("es-ES", {
                     maximumFractionDigits: 0,
                   })}
                   /BTC
@@ -164,7 +164,7 @@ export default function TradeDetailPage() {
                 <span style={styles.detailLabel}>Agreed Price:</span>
                 <span style={styles.detailValue}>
                   €
-                  {trade.agreed_price_eur.toLocaleString("de-DE", {
+                  {trade.agreed_price_eur.toLocaleString("es-ES", {
                     maximumFractionDigits: 0,
                   })}
                   /BTC