feat(web): add simple auth UI + enable CORS for testing

frontend/src/app/page.tsx

@@ -1,63 +1,46 @@
-import Image from "next/image";
+import Link from "next/link";
+
+const API_BASE = process.env.NEXT_PUBLIC_API_BASE ?? "http://localhost:8080";
+
+async function fetchHealth() {
+  try {
+    const r = await fetch(`${API_BASE}/api/health`, { cache: "no-store" });
+    if (!r.ok) return { ok: false, error: `HTTP ${r.status}` } as const;
+    return (await r.json()) as { ok: boolean; version?: string };
+  } catch (e: unknown) {
+    return { ok: false, error: String(e) } as const;
+  }
+}
+
+export default async function Home() {
+  const health = await fetchHealth();
 
-export default function Home() {
   return (
-    <div className="flex min-h-screen items-center justify-center bg-zinc-50 font-sans dark:bg-black">
-      <main className="flex min-h-screen w-full max-w-3xl flex-col items-center justify-between py-32 px-16 bg-white dark:bg-black sm:items-start">
-        <Image
-          className="dark:invert"
-          src="/next.svg"
-          alt="Next.js logo"
-          width={100}
-          height={20}
-          priority
-        />
-        <div className="flex flex-col items-center gap-6 text-center sm:items-start sm:text-left">
-          <h1 className="max-w-xs text-3xl font-semibold leading-10 tracking-tight text-black dark:text-zinc-50">
-            To get started, edit the page.tsx file.
-          </h1>
-          <p className="max-w-md text-lg leading-8 text-zinc-600 dark:text-zinc-400">
-            Looking for a starting point or more instructions? Head over to{" "}
-            <a
-              href="https://vercel.com/templates?framework=next.js&utm_source=create-next-app&utm_medium=appdir-template-tw&utm_campaign=create-next-app"
-              className="font-medium text-zinc-950 dark:text-zinc-50"
-            >
-              Templates
-            </a>{" "}
-            or the{" "}
-            <a
-              href="https://nextjs.org/learn?utm_source=create-next-app&utm_medium=appdir-template-tw&utm_campaign=create-next-app"
-              className="font-medium text-zinc-950 dark:text-zinc-50"
-            >
-              Learning
-            </a>{" "}
-            center.
-          </p>
+    <div className="min-h-screen bg-zinc-50 text-zinc-900">
+      <main className="mx-auto max-w-3xl px-6 py-12">
+        <h1 className="text-3xl font-semibold">CSP 在线练习平台（MVP）</h1>
+        <p className="mt-2 text-sm text-zinc-600">
+          API Base: <span className="font-mono">{API_BASE}</span>
+        </p>
+
+        <div className="mt-6 rounded-xl border bg-white p-5">
+          <h2 className="text-lg font-medium">后端状态</h2>
+          <pre className="mt-3 overflow-auto rounded-lg bg-zinc-900 p-3 text-xs text-zinc-100">
+            {JSON.stringify(health, null, 2)}
+          </pre>
         </div>
-        <div className="flex flex-col gap-4 text-base font-medium sm:flex-row">
-          <a
-            className="flex h-12 w-full items-center justify-center gap-2 rounded-full bg-foreground px-5 text-background transition-colors hover:bg-[#383838] dark:hover:bg-[#ccc] md:w-[158px]"
-            href="https://vercel.com/new?utm_source=create-next-app&utm_medium=appdir-template-tw&utm_campaign=create-next-app"
-            target="_blank"
-            rel="noopener noreferrer"
+
+        <div className="mt-6 flex gap-3">
+          <Link
+            className="rounded-lg bg-zinc-900 px-4 py-2 text-white hover:bg-zinc-800"
+            href="/auth"
           >
-            <Image
-              className="dark:invert"
-              src="/vercel.svg"
-              alt="Vercel logomark"
-              width={16}
-              height={16}
-            />
-            Deploy Now
-          </a>
-          <a
-            className="flex h-12 w-full items-center justify-center rounded-full border border-solid border-black/[.08] px-5 transition-colors hover:border-transparent hover:bg-black/[.04] dark:border-white/[.145] dark:hover:bg-[#1a1a1a] md:w-[158px]"
-            href="https://nextjs.org/docs?utm_source=create-next-app&utm_medium=appdir-template-tw&utm_campaign=create-next-app"
-            target="_blank"
-            rel="noopener noreferrer"
-          >
-            Documentation
-          </a>
+            注册 / 登录
+          </Link>
+        </div>
+
+        <div className="mt-10 text-sm text-zinc-500">
+          说明：当前前端仅用于验证注册/登录与基础连通性；题库/提交/比赛/判题会在后续迭代补齐。
         </div>
       </main>
     </div>