fix: correct private_mode_mode_secret key to private_mode_secret in app configuration

This commit updates the key 'private_mode_mode_secret' to 'private_mode_secret' in the configuration file `app.php`.

config/app.php

@@ -137,5 +137,5 @@
 
     'cdn_disk' => env('CDN_FILESYSTEM_DISK'),
 
-    'private_mode_mode_secret' => env('APP_PRIVATE_MODE_SECRET'),
+    'private_mode_secret' => env('APP_PRIVATE_MODE_SECRET'),
 ];