Update e2e tests and mocks for locale-based routing

- Adjusted assertions and navigation tests to include `/en` locale prefix for consistency.
- Updated next-intl and components-i18n mocks to support locale handling in tests.
- Renamed "Components" link and related references to "Design System" in homepage tests.
- Disabled typing delay in debounce test for improved test reliability.

frontend/tests/__mocks__/components-i18n.tsx

@@ -0,0 +1,5 @@
+/**
+ * Mock for @/components/i18n
+ */
+
+export const LocaleSwitcher = () => <div data-testid="locale-switcher">EN</div>;

frontend/tests/__mocks__/next-intl.tsx

@@ -0,0 +1,11 @@
+/**
+ * Mock for next-intl
+ */
+
+export const useLocale = jest.fn(() => 'en');
+export const useTranslations = jest.fn(() => (key: string) => key);
+export const NextIntlClientProvider = ({ children }: { children: React.ReactNode }) => <>{children}</>;
+export const useFormatter = jest.fn(() => ({}));
+export const useMessages = jest.fn(() => ({}));
+export const useNow = jest.fn(() => new Date());
+export const useTimeZone = jest.fn(() => 'UTC');