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/price-history.spec.ts

@@ -2,6 +2,12 @@ import { test, expect } from "@playwright/test";
 import { clearAuth, loginUser, REGULAR_USER, ADMIN_USER } from "./helpers/auth";
 
 test.describe("Price History - E2E", () => {
+  test.beforeEach(async ({ context }) => {
+    await context.addInitScript(() => {
+      window.localStorage.setItem("arbret-locale", "en");
+    });
+  });
+
   test("admin can view and use price history page, regular user cannot access", async ({
     page,
   }) => {