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

src/styles/global.css

@@ -17,6 +17,11 @@
   --dutch-blue: #1e4785;
 }
 
+/* Dark theme tokens */
+.dark {
+  --dutch-white: #0b0f14;
+}
+
 /* Custom scrollbar */
 ::-webkit-scrollbar {
   width: 8px;