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

[waterWang]

Summary

Adds a multi-stage Dockerfile for the frontend so the static site can be built and served consistently in containerized deployments (k8s / Helm already reference a payd-frontend deployment image).

Changes

• frontend/Dockerfile — two-stage build:
  • Build stage (node:20-alpine): npm ci then npm run build (tsc + vite build), with VITE_* ARGs exposed so runtime config (API URL, Stellar network) can be injected at build time
  • Runtime stage (nginx:1.27-alpine): serves /usr/share/nginx/html, exposes port 80 (matches the k8s deployment containerPort: 80), includes a healthcheck for the k8s probes
• frontend/nginx.conf — SPA routing support via try_files ... /index.html fallback, 1-year immutable caching for hashed static assets, no-cache for index.html so releases propagate immediately, gzip for text assets
• frontend/.dockerignore — keeps node_modules, dist, test artifacts, and local env files out of the build context

Acceptance Criteria

• Multi-stage: build stage + nginx runtime
• Image size < 50MB (nginx:alpine runtime is ~45MB; check docker image ls payd-frontend)
• Proper caching headers for static assets (immutable, 1y for hashed js/css)
• SPA routing support (nginx fallback to index.html)

Verification

cd frontend
docker build -t payd-frontend .
docker run -p 8080:80 payd-frontend

Then visit http://localhost:8080/ and any client-side route (e.g. /settings) — both should render the app.

Closes #539