fix(auth): support both server and public environment variables for guest credentials service

app/(auth)/login/page.tsx

@@ -1,6 +1,6 @@
 /**
  * @file (auth)/login/page.tsx
- * @description Client component providing a user login interface with NextAuth credentials authentication.
+ * @description Client component providing a user login interface with NextAuth credentials authentication and guest login capabilities.
  */
 
 "use client";
@@ -9,6 +9,7 @@ import { useState } from "react";
 import { signIn } from "next-auth/react";
 import { useRouter } from "next/navigation";
 import Link from "next/link";
+import { getGuestCredentials } from "@/services/auth.service";
 
 /**
  * Renders the login page containing the authentication form, error handling,
@@ -86,21 +87,35 @@ export default function LoginPage() {
   };
 
   /**
-   * Handles quick guest login using environment variables.
+   * Handles quick guest login securely via Server Action.
    *
    * @async
    * @returns {Promise<void>}
    */
   const handleGuestLogin = async () => {
-    const guestEmail = process.env.NEXT_PUBLIC_GUEST_EMAIL;
+    setError(null);
-    const guestPassword = process.env.NEXT_PUBLIC_GUEST_PASSWORD;
+    setLoadingType("guest");
 
-    if (!guestEmail || !guestPassword) {
+    try {
-      setError("Guest login is not configured.");
+      const result = await getGuestCredentials();
+
+      if ("error" in result && result.error) {
+        setError(result.error);
+        setLoadingType(null);
         return;
       }
 
-    await handleSignIn(guestEmail, guestPassword, "guest");
+      if (!result.email || !result.password) {
+        setError("Guest login is not configured properly.");
+        setLoadingType(null);
+        return;
+      }
+
+      await handleSignIn(result.email, result.password, "guest");
+    } catch (err) {
+      setError("Failed to initialize guest login.");
+      setLoadingType(null);
+    }
   };
 
   return (

services/auth.service.ts

@@ -0,0 +1,22 @@
+/**
+ * @file services/auth.service.ts
+ * @description Authentication service providing helper functions for retrieving authentication details such as guest credentials from environment variables.
+ */
+
+/**
+ * Retrieves configured guest account credentials from server or public environment variables.
+ *
+ * @async
+ * @returns {Promise<{ email?: string; password?: string; error?: string }>} An object containing the guest email and password, or an error message if unconfigured.
+ */
+export async function getGuestCredentials() {
+  const email = process.env.GUEST_EMAIL || process.env.NEXT_PUBLIC_GUEST_EMAIL;
+  const password =
+    process.env.GUEST_PASSWORD || process.env.NEXT_PUBLIC_GUEST_PASSWORD;
+
+  if (!email || !password) {
+    return { error: "Guest login is not configured on the server." };
+  }
+
+  return { email, password };
+}