feat: Add SMTP testing and improve Docker setup

Introduce a new feature to test SMTP configuration directly from the settings page. This involves adding a new API endpoint and corresponding UI elements to trigger and display the results of an SMTP test.

Additionally, this commit refactors the Docker setup by consolidating Dockerfiles and removing unnecessary configuration files. The goal is to streamline the build process and reduce image size and complexity.

nginx.conf

@@ -1,38 +0,0 @@
-worker_processes 1;
-
-events { worker_connections 1024; }
-
-http {
-    include       mime.types;
-    default_type  application/octet-stream;
-    sendfile        on;
-    keepalive_timeout  65;
-
-    server {
-        listen 80;
-        root /usr/share/nginx/html;
-        index index.html;
-
-        # Limite upload per allegati (es. foto/video ticket) - Allineato con il backend
-        client_max_body_size 50M;
-
-        # Compressione Gzip
-        gzip on;
-        gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
-
-        # Gestione SPA (React Router)
-        location / {
-            try_files $uri $uri/ /index.html;
-        }
-
-        # Proxy API verso il backend
-        location /api/ {
-            proxy_pass http://backend:3001;
-            proxy_http_version 1.1;
-            proxy_set_header Upgrade $http_upgrade;
-            proxy_set_header Connection 'upgrade';
-            proxy_set_header Host $host;
-            proxy_cache_bypass $http_upgrade;
-        }
-    }
-}