feat: add i18n with English (default) and PT-BR (/pt-br)

- Translate index.html to English as default language
- Copy PT-BR version to index.pt-br.html
- Add hreflang tags for SEO in both versions
- Add language selector in navbar (EN ↔ PT-BR)
- Add nginx.conf with Content-Language headers and /pt-br routing
- Update Dockerfile to process and serve both HTML files

nginx.conf

@@ -0,0 +1,24 @@
+server {
+    listen 80;
+    server_name _;
+    root /usr/share/nginx/html;
+
+    location / {
+        try_files $uri $uri/ =404;
+        default_type text/html;
+        add_header Content-Language "en" always;
+    }
+
+    location /pt-br {
+        try_files $uri $uri/ /pt-br/index.html =404;
+        default_type text/html;
+        add_header Content-Language "pt-BR" always;
+    }
+
+    location = /pt-br {
+        return 301 /pt-br/;
+    }
+
+    gzip on;
+    gzip_types text/html text/css application/javascript;
+}