feat: refactor layout to use SiteHeader component

components/site-header.tsx

@@ -0,0 +1,30 @@
+import Link from "next/link";
+
+export function SiteHeader() {
+  return (
+    <header className="sticky top-0 z-40 border-b bg-background/85 backdrop-blur motion-safe:animate-in motion-safe:fade-in motion-safe:slide-in-from-top-2 motion-safe:duration-500">
+      <div className="mx-auto flex w-full max-w-6xl items-center justify-between px-4 py-3 sm:px-8">
+        <Link
+          href="/"
+          className="text-sm font-semibold tracking-tight transition-colors duration-200 hover:text-primary sm:text-base"
+        >
+          Takasumi-Neodyマイクラサーバプロジェクト
+        </Link>
+        <nav className="flex items-center gap-4 text-sm">
+          <Link
+            href="/"
+            className="underline-offset-4 transition-colors duration-200 hover:text-primary hover:underline"
+          >
+            メインページ
+          </Link>
+          <Link
+            href="/announcements"
+            className="underline-offset-4 transition-colors duration-200 hover:text-primary hover:underline"
+          >
+            お知らせ
+          </Link>
+        </nav>
+      </div>
+    </header>
+  );
+}