feat: add multi-stage Dockerfile for frontend static site serving (Closes #539)

frontend/.dockerignore

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+node_modules
+dist
+e2e
+playwright-report
+test-results
+*.tsbuildinfo
+.env
+.env.*
+.DS_Store
+npm-debug.log*

frontend/Dockerfile

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+# Multi-stage build: Node build stage + nginx runtime stage
+# Docs: https://vite.dev/guide/static-deploy
+
+# ---------- Build stage ----------
+FROM node:20-alpine AS builder
+
+WORKDIR /app
+
+# Install dependencies first for better layer caching
+COPY package.json package-lock.json ./
+RUN npm ci
+
+# Copy source and build
+COPY . .
+
+# Build the static site (tsc + vite build)
+ARG VITE_API_URL
+ARG VITE_STELLAR_NETWORK
+ARG VITE_STELLAR_NETWORK_PASSPHRASE
+ARG VITE_STELLAR_HORIZON_URL
+ENV VITE_API_URL=${VITE_API_URL} \
+    VITE_STELLAR_NETWORK=${VITE_STELLAR_NETWORK} \
+    VITE_STELLAR_NETWORK_PASSPHRASE=${VITE_STELLAR_NETWORK_PASSPHRASE} \
+    VITE_STELLAR_HORIZON_URL=${VITE_STELLAR_HORIZON_URL}
+
+RUN npm run build
+
+# ---------- Runtime stage ----------
+FROM nginx:1.27-alpine AS production
+
+# SPA routing with fallback to index.html + caching headers
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+
+COPY --from=builder /app/dist /usr/share/nginx/html
+
+EXPOSE 80
+
+HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
+  CMD wget -qO- http://localhost/ > /dev/null || exit 1
+
+CMD ["nginx", "-g", "daemon off;"]

frontend/nginx.conf

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+server {
+    listen 80;
+    server_name _;
+
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # Gzip static assets
+    gzip on;
+    gzip_types text/plain text/css application/javascript application/json image/svg+xml;
+    gzip_min_length 1024;
+
+    # Hashed assets (js/css with content hash) — cache forever
+    location ~* \.(?:js|css|png|jpg|jpeg|gif|webp|avif|svg|ico|woff2?)$ {
+        expires 1y;
+        add_header Cache-Control "public, max-age=31536000, immutable";
+        try_files $uri =404;
+    }
+
+    # index.html — never cache so deploys propagate immediately
+    location = /index.html {
+        add_header Cache-Control "no-cache, no-store, must-revalidate";
+    }
+
+    # SPA routing — fall back to index.html for client-side routes
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+}