Refactor authentication redirection logic

- Introduce a new `auth-redirect` module to manage safe return paths for login and redirection.
- Update the login page to utilize `readReturnToFromLocation` for redirecting users post-login.
- Modify the `AuthGate` component to use `loginPathWithReturnTo` for redirecting unauthorized users, enhancing security and user experience.
- Adjust API request handling to incorporate the new redirection logic, ensuring users are directed to the appropriate login page with return paths.

frontend/src/app/login/page.tsx

@@ -4,6 +4,7 @@ import { FormEvent, useEffect, useState } from "react";
 import { useRouter } from "next/navigation";
 import { LockKeyhole, LogIn, Video } from "lucide-react";
 import { useAuth } from "@/components/auth/AuthProvider";
+import { readReturnToFromLocation } from "@/lib/auth-redirect";
 import { Button } from "@/components/ui/button";
 import {
   Card,
@@ -24,7 +25,7 @@ export default function LoginPage() {
 
   useEffect(() => {
     if (!loading && user) {
-      router.replace("/");
+      router.replace(readReturnToFromLocation());
     }
   }, [loading, router, user]);
 
@@ -34,7 +35,7 @@ export default function LoginPage() {
     setSubmitting(true);
     try {
       await login(username.trim(), password);
-      router.replace("/");
+      router.replace(readReturnToFromLocation());
     } catch (err) {
       setError(err instanceof Error ? err.message : "登录失败");
     } finally {