Add user authentication and role management

app/login/page.tsx

@@ -0,0 +1,69 @@
+"use client";
+
+import { FormEvent, useState } from "react";
+
+export default function LoginPage() {
+  const [username, setUsername] = useState("");
+  const [password, setPassword] = useState("");
+  const [error, setError] = useState("");
+  const [working, setWorking] = useState(false);
+
+  const submit = async (event: FormEvent) => {
+    event.preventDefault();
+    setError("");
+    setWorking(true);
+    try {
+      const response = await fetch("/api/auth/login", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ username, password }),
+      });
+      const result = (await response.json()) as { error?: string };
+      if (!response.ok) throw new Error(result.error || "登录失败");
+      window.location.assign("/");
+    } catch (reason) {
+      setError(reason instanceof Error ? reason.message : "登录失败");
+    } finally {
+      setWorking(false);
+    }
+  };
+
+  return (
+    <main className="login-page">
+      <section className="login-brand">
+        <div className="login-logo">K</div>
+        <p>KOC LOOP</p>
+        <h1>内容分发闭环</h1>
+        <span>统一管理内容任务、KOC 发布和数据回收。</span>
+      </section>
+      <form className="login-card" onSubmit={submit}>
+        <p className="eyebrow">后台登录</p>
+        <h2>欢迎回来</h2>
+        <span className="login-tip">请使用管理员分配的账号和密码</span>
+        <label>
+          <span>登录账号</span>
+          <input
+            value={username}
+            onChange={(event) => setUsername(event.target.value)}
+            autoComplete="username"
+            required
+          />
+        </label>
+        <label>
+          <span>密码</span>
+          <input
+            type="password"
+            value={password}
+            onChange={(event) => setPassword(event.target.value)}
+            autoComplete="current-password"
+            required
+          />
+        </label>
+        {error && <p className="login-error">{error}</p>}
+        <button className="primary-button" disabled={working}>
+          {working ? "登录中…" : "登录"}
+        </button>
+      </form>
+    </main>
+  );
+}