Set up initial frontend with Vite and integrated Docker for full-stack build

backend/Dockerfile

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+# Build context : racine du projet (budget-commun/)
+
+# ── Stage 1 : Build React ─────────────────────────────────────
+FROM node:20-alpine AS frontend-builder
+WORKDIR /build
+COPY frontend/package.json .
+RUN npm install
+COPY frontend .
+RUN npx vite build --outDir /dist --emptyOutDir
+
+# ── Stage 2 : Backend Node.js ──────────────────────────────────
+FROM node:20-alpine
+WORKDIR /app
+COPY backend/package.json .
+RUN npm install --production
+COPY backend .
+COPY --from=frontend-builder /dist ./public
+EXPOSE 3000
+CMD ["node", "server.js"]