Add React frontend and Sinbad2IA LLM integration.

Introduce a full Vite/React UI for exams, auth, materials, images, generation, and export.
Adapt backend for Sinbad2IA chat API, bcrypt passwords, CORS on port 5173, and schema migrations.

frontend/nginx.conf

@@ -0,0 +1,21 @@
+server {
+    listen 80;
+    server_name _;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # SPA: cualquier ruta desconocida sirve index.html (React Router).
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+
+    # Cache de assets con hash.
+    location /assets/ {
+        expires 1y;
+        add_header Cache-Control "public, immutable";
+    }
+
+    gzip on;
+    gzip_types text/css application/javascript application/json image/svg+xml;
+    gzip_min_length 1024;
+}