feat: add Dockerfile + nginx config for Coolify deployment

Multi-stage build: Flutter stable builds web release, nginx:alpine serves it.
nginx.conf handles SPA routing (all paths → index.html) and caches assets.

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

Dockerfile

@@ -0,0 +1,18 @@
+# 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 . .
+RUN flutter build web --release --base-href /
+
+# Stage 2: Serve with nginx
+FROM nginx:alpine
+
+COPY --from=builder /app/build/web /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+
+EXPOSE 80
+CMD ["nginx", "-g", "daemon off;"]