feat(docker): ajout service frontend Flutter Web (Nginx port 3001)

- frontend/Dockerfile : build multi-étapes (Flutter stable → Nginx alpine)
- frontend/nginx.conf : SPA routing, cache assets, gzip
- docker-compose.yml : service frontend sur port 3001
- Accessible via reverse proxy → https://app.rayhan.bolbol.tn

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/Dockerfile

@@ -0,0 +1,13 @@
+# Étape 1 : Build Flutter Web
+FROM ghcr.io/cirruslabs/flutter:stable AS build
+WORKDIR /app
+COPY pubspec.yaml pubspec.lock* ./
+RUN flutter pub get
+COPY . .
+RUN flutter build web --release --base-href /
+
+# Étape 2 : Serveur Nginx
+FROM nginx:alpine
+COPY --from=build /app/build/web /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+EXPOSE 80