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/admin-invites.spec.ts

@@ -16,7 +16,10 @@ async function loginAsAdmin(page: Page) {
 }
 
 test.describe("Admin Invites Page", () => {
-  test.beforeEach(async ({ page }) => {
+  test.beforeEach(async ({ context, page }) => {
+    await context.addInitScript(() => {
+      window.localStorage.setItem("arbret-locale", "en");
+    });
     await page.context().clearCookies();
     await loginAsAdmin(page);
   });