Add Dockerfile, frontend image, and local compose stack

frontend/Dockerfile

@@ -0,0 +1,22 @@
+# Shattered Void MMO - frontend (Vite/React build served by nginx)
+
+FROM node:20-bookworm-slim AS build
+WORKDIR /app
+
+COPY package.json package-lock.json ./
+RUN npm ci
+
+COPY . .
+
+# Vite inlines env vars at build time, so the API URL must be known here, not at runtime.
+ARG VITE_API_URL=http://localhost:3000
+ARG VITE_WS_URL=http://localhost:3000
+ENV VITE_API_URL=$VITE_API_URL
+ENV VITE_WS_URL=$VITE_WS_URL
+
+RUN npm run build
+
+FROM nginx:1.27-alpine AS runtime
+COPY --from=build /app/dist /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+EXPOSE 80