feat: harden bootstrap and add tests

tests/language-switcher.test.tsx

@@ -0,0 +1,21 @@
+import { beforeEach, describe, expect, it } from 'vitest';
+import { fireEvent, render, screen } from '@testing-library/react';
+import LanguageSwitcher from '@/components/LanguageSwitcher';
+
+describe('LanguageSwitcher', () => {
+  beforeEach(() => {
+    localStorage.clear();
+  });
+
+  it('switches language without reloading the page', () => {
+    localStorage.setItem('language', 'en');
+
+    render(<LanguageSwitcher />);
+
+    const button = screen.getByRole('button', { name: 'Switch to Spanish' });
+    fireEvent.click(button);
+
+    expect(localStorage.getItem('language')).toBe('es');
+    expect(screen.getByRole('button', { name: 'Switch to English' })).toBeInTheDocument();
+  });
+});