fix: persist dark mode theme across page reloads

- Save theme preference to localStorage on toggle
- Load from localStorage or system preference on mount
- Add blocking script to prevent flash of wrong theme

components/Header.tsx

@@ -28,13 +28,28 @@ export default function Header() {
   }
 
   useEffect(() => {
-    const isDark = document.documentElement.classList.contains('dark')
+    const stored = localStorage.getItem('theme')
+    const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches
+    const isDark = stored === 'dark' || (!stored && prefersDark)
+    
+    if (isDark) {
+      document.documentElement.classList.add('dark')
+    } else {
+      document.documentElement.classList.remove('dark')
+    }
     setDark(isDark)
   }, [])
 
   const toggleDark = () => {
-    document.documentElement.classList.toggle('dark')
-    setDark(!dark)
+    const newDark = !dark
+    if (newDark) {
+      document.documentElement.classList.add('dark')
+      localStorage.setItem('theme', 'dark')
+    } else {
+      document.documentElement.classList.remove('dark')
+      localStorage.setItem('theme', 'light')
+    }
+    setDark(newDark)
   }
 
   return (