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/login/page.tsx

@@ -1,14 +1,50 @@
-import { Suspense } from "react";
+"use client";
+
+import { Suspense, useEffect } from "react";
+import { useRouter, useSearchParams } from "next/navigation";
+import {
+  Dialog,
+  DialogContent,
+  DialogHeader,
+  DialogTitle,
+} from "@/components/ui/dialog";
 import { LoginForm } from "./login-form";
 
 const hasKeycloak = !!process.env.AUTH_KEYCLOAK_ISSUER;
 
-export default function LoginPage() {
+function LoginPageContent() {
+  const router = useRouter();
+  const searchParams = useSearchParams();
+  const callbackUrl = searchParams.get("callbackUrl");
+  const error = searchParams.get("error");
+
+  useEffect(() => {
+    // If not an OAuth callback, redirect to home with login modal open
+    if (!callbackUrl && !error) {
+      router.replace("/?login=1");
+    }
+  }, [callbackUrl, error, router]);
+
   return (
-    <div className="flex min-h-screen items-center justify-center bg-gray-50 p-4">
-      <Suspense fallback={<div>加载中...</div>}>
-        <LoginForm hasKeycloak={hasKeycloak} />
-      </Suspense>
+    <div className="flex min-h-screen items-center justify-center bg-gradient-to-br from-slate-50 to-slate-100 p-4">
+      <Dialog defaultOpen>
+        <DialogContent className="sm:max-w-md">
+          <DialogHeader>
+            <DialogTitle className="text-center">登录</DialogTitle>
+          </DialogHeader>
+          <Suspense fallback={<div>加载中...</div>}>
+            <LoginForm hasKeycloak={hasKeycloak} />
+          </Suspense>
+        </DialogContent>
+      </Dialog>
     </div>
   );
 }
+
+export default function LoginPage() {
+  return (
+    <Suspense fallback={<div className="flex min-h-screen items-center justify-center">加载中...</div>}>
+      <LoginPageContent />
+    </Suspense>
+  );
+}