feat: site-wide dark mode (Tailwind class strategy); ThemeToggle island in header; persist preference; adjust brand-surface for dark

tailwind.config.js

@@ -1,6 +1,7 @@
 // tailwind.config.mjs
 /** @type {import('tailwindcss').Config} */
 export default {
+  darkMode: 'class',
   content: ['./src/**/*.{astro,html,js,jsx,md,mdx,svelte,ts,tsx,vue}'],
   theme: {
     extend: {