frontend: convert login and register to homepage modals

- Remove register link from login form
- Redirect /login and /register to /?login=1 and /?register=1
- Open login/register as dialogs on homepage instead of separate pages

frontend/app/register/page.tsx

@@ -1,113 +1,18 @@
 "use client";
 
-import { useState } from "react";
+import { useEffect } from "react";
 import { useRouter } from "next/navigation";
-import { Button } from "@/components/ui/button";
-import { Input } from "@/components/ui/input";
-import { Label } from "@/components/ui/label";
-import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/card";
 
 export default function RegisterPage() {
   const router = useRouter();
-  const [form, setForm] = useState({
-    username: "",
-    email: "",
-    password: "",
-    confirmPassword: "",
-  });
-  const [error, setError] = useState("");
 
-  async function handleSubmit(e: React.FormEvent) {
-    e.preventDefault();
-    setError("");
-
-    if (form.password !== form.confirmPassword) {
-      setError("两次输入的密码不一致");
-      return;
-    }
-
-    const res = await fetch("/api/proxy/auth/register", {
-      method: "POST",
-      headers: { "Content-Type": "application/json" },
-      body: JSON.stringify({
-        username: form.username,
-        email: form.email,
-        password: form.password,
-      }),
-    });
-
-    if (!res.ok) {
-      const data = await res.json();
-      setError(data.error || "注册失败");
-      return;
-    }
-
-    router.push("/login");
-  }
+  useEffect(() => {
+    router.replace("/");
+  }, [router]);
 
   return (
-    <div className="flex min-h-screen items-center justify-center bg-gray-50 p-4">
-      <Card className="w-full max-w-md">
-        <CardHeader>
-          <CardTitle className="text-center">注册</CardTitle>
-        </CardHeader>
-        <CardContent className="space-y-4">
-          <form onSubmit={handleSubmit} className="space-y-4">
-            <div>
-              <Label htmlFor="username">用户名</Label>
-              <Input
-                id="username"
-                value={form.username}
-                onChange={(e) => setForm({ ...form, username: e.target.value })}
-                required
-              />
-            </div>
-            <div>
-              <Label htmlFor="email">邮箱</Label>
-              <Input
-                id="email"
-                type="email"
-                value={form.email}
-                onChange={(e) => setForm({ ...form, email: e.target.value })}
-                required
-              />
-            </div>
-            <div>
-              <Label htmlFor="password">密码</Label>
-              <Input
-                id="password"
-                type="password"
-                value={form.password}
-                onChange={(e) => setForm({ ...form, password: e.target.value })}
-                required
-              />
-            </div>
-            <div>
-              <Label htmlFor="confirmPassword">确认密码</Label>
-              <Input
-                id="confirmPassword"
-                type="password"
-                value={form.confirmPassword}
-                onChange={(e) =>
-                  setForm({ ...form, confirmPassword: e.target.value })
-                }
-                required
-              />
-            </div>
-            {error && <p className="text-sm text-red-500">{error}</p>}
-            <Button type="submit" className="w-full">
-              注册
-            </Button>
-          </form>
-
-          <p className="text-center text-sm text-gray-500">
-            已有账号？{" "}
-            <a href="/login" className="text-blue-600 hover:underline">
-              去登录
-            </a>
-          </p>
-        </CardContent>
-      </Card>
+    <div className="flex min-h-screen items-center justify-center">
+      加载中...
     </div>
   );
 }