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/e2e/availability.spec.ts

@@ -12,11 +12,14 @@ import { API_URL, REGULAR_USER, ADMIN_USER, clearAuth, loginUser } from "./helpe
 function getTomorrowDisplay(): string {
   const tomorrow = new Date();
   tomorrow.setDate(tomorrow.getDate() + 1);
-  return tomorrow.toLocaleDateString("en-US", { weekday: "short", month: "short", day: "numeric" });
+  return tomorrow.toLocaleDateString("es-ES", { weekday: "short", month: "short", day: "numeric" });
 }
 
 test.describe("Availability Page - Admin Access", () => {
-  test.beforeEach(async ({ page }) => {
+  test.beforeEach(async ({ context, page }) => {
+    await context.addInitScript(() => {
+      window.localStorage.setItem("arbret-locale", "en");
+    });
     await clearAuth(page);
     await loginUser(page, ADMIN_USER.email, ADMIN_USER.password);
   });