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.

vitest.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    fileParallelism: false,
+  },
+});