feat(frontend): add nginx production container

.github/workflows/frontend-container.yml

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+name: Frontend container
+
+on:
+  push:
+    paths:
+      - 'frontend/**'
+      - '.github/workflows/frontend-container.yml'
+  pull_request:
+    paths:
+      - 'frontend/**'
+      - '.github/workflows/frontend-container.yml'
+
+permissions:
+  contents: read
+
+concurrency:
+  group: frontend-container-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  container:
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+      - name: Build frontend image
+        run: docker build --pull -t payd-frontend:ci -f frontend/Dockerfile frontend
+      - name: Check image size and nginx configuration
+        run: |
+          bytes=$(docker image inspect payd-frontend:ci --format '{{.Size}}')
+          echo "Frontend image size: ${bytes} bytes"
+          test "$bytes" -lt 50000000
+          docker run --rm payd-frontend:ci nginx -t
+      - name: Check static serving and SPA fallback
+        shell: bash
+        run: |
+          set -euo pipefail
+          container=$(docker run -d -p 127.0.0.1:8080:80 payd-frontend:ci)
+          trap 'docker rm -f "$container" >/dev/null' EXIT
+          for attempt in $(seq 1 30); do
+            if curl -fsS http://127.0.0.1:8080/ -o /tmp/index.html; then break; fi
+            sleep 1
+          done
+          curl -fsS -D /tmp/index.headers http://127.0.0.1:8080/index.html -o /tmp/index.html
+          grep -qi '^Cache-Control: no-store' /tmp/index.headers
+          curl -fsS -D /tmp/route.headers http://127.0.0.1:8080/dashboard -o /tmp/route.html
+          cmp /tmp/index.html /tmp/route.html
+          grep -qi '^Cache-Control: no-store' /tmp/route.headers
+          asset=$(docker exec "$container" find /usr/share/nginx/html/assets -type f -name '*.js' | head -n 1)
+          test -n "$asset"
+          asset=${asset#/usr/share/nginx/html}
+          curl -fsS -D /tmp/asset.headers "http://127.0.0.1:8080${asset}" -o /dev/null
+          grep -qi '^Cache-Control: public, max-age=31536000, immutable' /tmp/asset.headers
+          status=$(curl -sS -D /tmp/missing.headers -o /dev/null -w '%{http_code}' http://127.0.0.1:8080/assets/missing-container-smoke.js)
+          test "$status" = 404
+          ! grep -qi 'immutable' /tmp/missing.headers

frontend/.dockerignore

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+.git
+**/node_modules
+**/dist
+**/.vite
+**/coverage
+playwright-report
+test-results
+*.log
+.env
+.env.*

frontend/DOCKER.md

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+# Frontend container
+
+From the repository root:
+
+```sh
+docker build --pull -t payd-frontend:local -f frontend/Dockerfile frontend
+docker run --rm -p 8080:80 payd-frontend:local
+```
+
+The Node 22 build stage runs the existing locked dependency install and
+`npm run build`; only Vite's `dist` output enters the nginx Alpine slim runtime.
+Port 80 matches the Kubernetes and Helm frontend service/probe configuration.
+Build errors are not suppressed. No backend, database, or provider runs in this
+image: route API requests to the backend through the existing ingress. Vite's
+local development proxy is not part of a production static server.
+
+Vite embeds public configuration at build time. Pass the required
+`PUBLIC_STELLAR_NETWORK`, `PUBLIC_STELLAR_NETWORK_PASSPHRASE`,
+`PUBLIC_STELLAR_RPC_URL`, and `PUBLIC_STELLAR_HORIZON_URL` using `--build-arg`.
+Do not pass secrets as build arguments. Local `.env` files are excluded from the
+build context; runtime `docker run -e` values do not rewrite the built application.
+
+## Focused acceptance checks
+
+```sh
+# Issue #539 requires the uncompressed local image to be below 50 MB.
+bytes=$(docker image inspect payd-frontend:local --format '{{.Size}}')
+test "$bytes" -lt 50000000
+
+docker run --rm payd-frontend:local nginx -t
+curl -fsSI http://localhost:8080/index.html  # Cache-Control: no-store
+curl -fsSI http://localhost:8080/dashboard  # SPA fallback; no-store
+curl -sSI http://localhost:8080/assets/missing.js  # 404; no immutable cache
+# Request a real /assets/<hashed-name>.js from index.html:
+# Cache-Control: public, max-age=31536000, immutable
+```
+
+`index.html` and SPA fallbacks are never stored. Vite's hashed `/assets/` files
+are immutable for one year; other public files must revalidate. Check the image
+size after building, since application assets and upstream image tags can grow.

frontend/Dockerfile

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+# Build from the repository root:
+# docker build -t payd-frontend -f frontend/Dockerfile frontend
+FROM node:22-alpine AS build
+WORKDIR /app
+
+# The wallet dependency tree includes node-usb, which builds from source on musl.
+# These tools and headers remain in the build stage, not the nginx image.
+RUN apk add --no-cache python3 make g++ pkgconf eudev-dev linux-headers
+
+# Copy the workspace sources before npm ci so local packages are available too.
+COPY . .
+RUN npm ci --legacy-peer-deps
+
+# These are public browser configuration, never credentials. Vite substitutes
+# them at build time; setting container environment variables cannot change dist.
+ARG PUBLIC_STELLAR_NETWORK
+ARG PUBLIC_STELLAR_NETWORK_PASSPHRASE
+ARG PUBLIC_STELLAR_RPC_URL
+ARG PUBLIC_STELLAR_HORIZON_URL
+RUN npm run build
+
+FROM nginx:stable-alpine-slim AS runtime
+RUN rm -rf /usr/share/nginx/html/*
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+COPY --from=build /app/dist/ /usr/share/nginx/html/
+EXPOSE 80
+CMD ["nginx", "-g", "daemon off;"]

frontend/nginx.conf

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+server {
+    listen 80;
+    listen [::]:80;
+    server_name _;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # Vite fingerprints these files. Missing assets must not become HTML or
+    # receive a long-lived cache header.
+    location /assets/ {
+        try_files $uri =404;
+        add_header Cache-Control "public, max-age=31536000, immutable";
+    }
+
+    # The entry point must always reflect the currently deployed asset hashes.
+    # Internal SPA redirects also pass through this exact location.
+    location = /index.html {
+        add_header Cache-Control "no-store" always;
+    }
+
+    # Public files outside /assets are not necessarily content-addressed.
+    location / {
+        try_files $uri $uri/ /index.html;
+        add_header Cache-Control "no-cache" always;
+    }
+}