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/nginx.conf

@@ -0,0 +1,22 @@
+server {
+    listen 8080;
+
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # SPA fallback: all unmatched paths → app shell
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+
+    # Long-lived cache for fingerprinted assets
+    location ~* \.(js|css|woff2?|ttf|eot|svg|png|jpg|jpeg|gif|ico)$ {
+        expires 1y;
+        add_header Cache-Control "public, immutable";
+    }
+
+    # Never cache the entry point
+    location = /index.html {
+        add_header Cache-Control "no-store, no-cache, must-revalidate";
+    }
+}