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/admin/invites/page.tsx

@@ -116,7 +116,7 @@ export default function AdminInvitesPage() {
   };
 
   const formatDate = (dateStr: string) => {
-    return new Date(dateStr).toLocaleString();
+    return new Date(dateStr).toLocaleString("es-ES");
   };
 
   const getStatusBadgeVariant = (status: string): "ready" | "success" | "error" | undefined => {

frontend/app/admin/price-history/page.tsx

@@ -39,11 +39,11 @@ export default function AdminPriceHistoryPage() {
   };
 
   const formatDate = (dateStr: string) => {
-    return new Date(dateStr).toLocaleString();
+    return new Date(dateStr).toLocaleString("es-ES");
   };
 
   const formatPrice = (price: number) => {
-    return new Intl.NumberFormat("en-US", {
+    return new Intl.NumberFormat("es-ES", {
       style: "currency",
       currency: "EUR",
       minimumFractionDigits: 2,

frontend/app/admin/trades/page.tsx

@@ -262,7 +262,7 @@ export default function AdminTradesPage() {
                         <span style={tradeCardStyles.rateLabel}>Rate:</span>
                         <span style={tradeCardStyles.rateValue}>
                           €
-                          {trade.agreed_price_eur.toLocaleString("de-DE", {
+                          {trade.agreed_price_eur.toLocaleString("es-ES", {
                             maximumFractionDigits: 0,
                           })}
                           /BTC
@@ -270,7 +270,7 @@ export default function AdminTradesPage() {
                         <span style={tradeCardStyles.rateLabel}>Market:</span>
                         <span style={tradeCardStyles.rateValue}>
                           €
-                          {trade.market_price_eur.toLocaleString("de-DE", {
+                          {trade.market_price_eur.toLocaleString("es-ES", {
                             maximumFractionDigits: 0,
                           })}
                         </span>