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/components/auth/AuthGate.tsx

@@ -4,6 +4,7 @@ import type { ReactNode } from "react";
 import { useEffect } from "react";
 import { usePathname, useRouter } from "next/navigation";
 import { AppShell } from "@/components/layout/AppShell";
+import { loginPathWithReturnTo } from "@/lib/auth-redirect";
 import { useAuth } from "./AuthProvider";
 
 export function AuthGate({ children }: { children: ReactNode }) {
@@ -15,9 +16,9 @@ export function AuthGate({ children }: { children: ReactNode }) {
 
   useEffect(() => {
     if (!loading && !user && !isLoginPage) {
-      router.replace("/login");
+      router.replace(loginPathWithReturnTo(pathname));
     }
-  }, [isLoginPage, loading, router, user]);
+  }, [isLoginPage, loading, pathname, router, user]);
 
   if (isLoginPage) {
     return <>{children}</>;