Feat: Add production-grade multi-stage container image for UI

Two-stage build (node:22-slim builder + nginxinc/nginx-unprivileged:alpine
runtime) with SPA fallback routing, long-lived cache headers for fingerprinted
assets, non-root user (UID 101), and no Node.js toolchain in runtime image
(82 MB vs 329 MB+ single-stage). Verified by ui/tests/build/verify_production_image.sh
covering build, health, SPA routing, non-root, stdout logging, cache-control
headers, SIGTERM exit 0, Node.js absent, secret-free layers, and dep-layer
cache hit. 102 integration tests still pass; shellcheck clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/.dockerignore

@@ -7,3 +7,5 @@ coverage/
 .env.*
 !.env.example
 *.log
+*.spec.ts
+tests/