Add Dockerfile, frontend image, and local compose stack

frontend/.dockerignore

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+node_modules
+dist
+.git
+*.md
+Dockerfile
+.dockerignore

frontend/Dockerfile

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+# 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

frontend/nginx.conf

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+server {
+    listen 80;
+    server_name _;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # React Router owns the routing; every unknown path must fall through to the SPA.
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+
+    location /healthz {
+        access_log off;
+        return 200 "ok\n";
+    }
+
+    gzip on;
+    gzip_types text/css application/javascript application/json image/svg+xml;
+    gzip_min_length 1024;
+}