feat: add Flutter web frontend with authentication and recipe management features

flutter/Dockerfile

@@ -0,0 +1,26 @@
+# Stage 1 – Build Flutter web
+FROM ghcr.io/cirruslabs/flutter:stable AS builder
+
+WORKDIR /app
+
+COPY pubspec.yaml pubspec.lock* ./
+RUN flutter pub get
+
+COPY . .
+
+# Inject the internal API URL at build time via --dart-define
+ARG FLUTTER_API_URL_INTERNAL=http://recipe-api:8080
+RUN flutter build web --release \
+    --dart-define=API_BASE_URL=${FLUTTER_API_URL_INTERNAL}
+
+# Stage 2 – Serve with Caddy
+FROM caddy:alpine AS runner
+
+ARG PORT=5000
+ENV PORT=${PORT}
+
+COPY --from=builder /app/build/web /usr/share/caddy
+COPY Caddyfile /etc/caddy/Caddyfile
+
+EXPOSE ${PORT}
+CMD ["caddy", "run", "--config", "/etc/caddy/Caddyfile", "--adapter", "caddyfile"]