Add Dockerfile, frontend image, and local compose stack

frontend/nginx.conf

@@ -0,0 +1,20 @@
+server {
+    listen 80;
+    server_name _;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # React Router owns the routing; every unknown path must fall through to the SPA.
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+
+    location /healthz {
+        access_log off;
+        return 200 "ok\n";
+    }
+
+    gzip on;
+    gzip_types text/css application/javascript application/json image/svg+xml;
+    gzip_min_length 1024;
+}