feat: add Header with language switcher and theme toggle

Also disables Vitest file parallelism since concurrent astro builds
across test files raced on the shared dist/ output.

tests/header.test.ts

@@ -0,0 +1,35 @@
+import { describe, it, expect, beforeAll } from 'vitest';
+import { execSync } from 'node:child_process';
+import { readFileSync } from 'node:fs';
+
+describe('Header, language switcher, theme toggle', () => {
+  beforeAll(() => {
+    execSync('npm run build', { stdio: 'inherit' });
+  });
+
+  it('renders the brand link on both locales', () => {
+    const en = readFileSync('dist/en/index.html', 'utf-8');
+    const fr = readFileSync('dist/fr/index.html', 'utf-8');
+    expect(en).toContain('class="brand" href="/en/"');
+    expect(fr).toContain('class="brand" href="/fr/"');
+  });
+
+  it('language switcher on /en/ links to /fr/ and vice versa', () => {
+    const en = readFileSync('dist/en/index.html', 'utf-8');
+    const fr = readFileSync('dist/fr/index.html', 'utf-8');
+    expect(en).toContain('class="language-switcher" href="/fr/"');
+    expect(fr).toContain('class="language-switcher" href="/en/"');
+  });
+
+  it('theme toggle button carries localized aria-labels', () => {
+    const en = readFileSync('dist/en/index.html', 'utf-8');
+    const fr = readFileSync('dist/fr/index.html', 'utf-8');
+    expect(en).toContain('aria-label="Switch to dark mode"');
+    expect(fr).toContain('aria-label="Passer en mode sombre"');
+  });
+
+  it('theme toggle click handler flips data-theme and persists to localStorage', () => {
+    const en = readFileSync('dist/en/index.html', 'utf-8');
+    expect(en).toContain("localStorage.setItem('ombrora-theme', next)");
+  });
+});