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/Dockerfile

@@ -0,0 +1,20 @@
+# --- Build stage ---
+FROM node:20-alpine AS build
+WORKDIR /app
+
+ARG VITE_API_URL=http://localhost:8000
+ARG VITE_GOOGLE_CLIENT_ID=
+ENV VITE_API_URL=$VITE_API_URL
+ENV VITE_GOOGLE_CLIENT_ID=$VITE_GOOGLE_CLIENT_ID
+
+COPY package.json package-lock.json* ./
+RUN npm install
+COPY . .
+RUN npm run build
+
+# --- Serve stage ---
+FROM nginx:1.27-alpine AS serve
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+COPY --from=build /app/dist /usr/share/nginx/html
+EXPOSE 80
+CMD ["nginx", "-g", "daemon off;"]