Fix: increase PHP and Nginx upload limit to 512M for backup restore and enhance upload error reporting

src/FirewallManager.php

@@ -380,7 +380,7 @@ server {
     listen [::]:80 default_server;
     server_name _;
 
-    client_max_body_size 64M;
+    client_max_body_size 512M;
 
     root /var/www/nexus-pbx/public;
     index index.php;
@@ -429,7 +429,7 @@ server {
     ssl_ciphers HIGH:!aNULL:!MD5;
     ssl_prefer_server_ciphers on;
 
-    client_max_body_size 64M;
+    client_max_body_size 512M;
 
     root /var/www/nexus-pbx/public;
     index index.php;