Add login gate, calculation history, and AI markdown download.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/login/page.tsx

@@ -0,0 +1,19 @@
+import { Suspense } from "react";
+import PageShell from "@/components/page-shell";
+import LoginForm from "@/components/auth/login-form";
+
+export default function LoginPage() {
+  return (
+    <PageShell width="narrow" className="py-12">
+      <div className="mb-8 text-center">
+        <h1 className="text-2xl font-bold tracking-wide">登录知命阁</h1>
+        <p className="mt-2 text-sm text-muted-foreground">
+          测算功能需登录后使用
+        </p>
+      </div>
+      <Suspense fallback={<p className="text-center text-sm">加载中…</p>}>
+        <LoginForm />
+      </Suspense>
+    </PageShell>
+  );
+}