fix(header): sync theme icon state after mount

- initialize dark mode state after mount from document to avoid mismatched first render state.
- render a placeholder before mount to reduce theme icon hydration mismatch risk.

web/src/components/layout/header.tsx

@@ -4,7 +4,7 @@ import Link from "next/link";
 import { usePathname } from "next/navigation";
 import { useTranslations, useLocale } from "@/lib/i18n";
 import { Github, Menu, X, Sun, Moon } from "lucide-react";
-import { useState } from "react";
+import { useState, useEffect } from "react";
 import { cn } from "@/lib/utils";
 
 const NAV_ITEMS = [
@@ -24,14 +24,13 @@ export function Header() {
   const pathname = usePathname();
   const locale = useLocale();
   const [mobileOpen, setMobileOpen] = useState(false);
-  const [dark, setDark] = useState(() => {
-    if (typeof window !== "undefined") {
-      const stored = localStorage.getItem("theme");
-      if (stored) return stored === "dark";
-      return window.matchMedia("(prefers-color-scheme: dark)").matches;
-    }
-    return false;
-  });
+  const [dark, setDark] = useState(false);
+  const [mounted, setMounted] = useState(false);
+
+  useEffect(() => {
+    setMounted(true);
+    setDark(document.documentElement.classList.contains("dark"));
+  }, []);
 
   function toggleDark() {
     const next = !dark;
@@ -91,7 +90,7 @@ export function Header() {
             onClick={toggleDark}
             className="rounded-md p-1.5 text-zinc-500 hover:text-zinc-700 dark:text-zinc-400 dark:hover:text-white"
           >
-            {dark ? <Sun size={16} /> : <Moon size={16} />}
+            {mounted ? (dark ? <Sun size={16} /> : <Moon size={16} />) : <span className="w-4 h-4 inline-block" />}
           </button>
 
           <a
@@ -148,7 +147,7 @@ export function Header() {
                 onClick={toggleDark}
                 className="flex min-h-[44px] min-w-[44px] items-center justify-center rounded-md text-zinc-500 hover:text-zinc-700 dark:text-zinc-400 dark:hover:text-white"
               >
-                {dark ? <Sun size={18} /> : <Moon size={18} />}
+                {mounted ? (dark ? <Sun size={18} /> : <Moon size={18} />) : <span className="w-[18px] h-[18px] inline-block" />}
               </button>
               <a
                 href="https://github.com/shareAI-lab/learn-claude-code"