fix: copy webchat UI assets to dist/ during build

tsc only compiles .ts files — the webchat static files (HTML, CSS, JS)
in src/gateway/ui/ were never copied to dist/gateway/ui/, causing 404s
when running the production build via 'pnpm start'.

package.json

@@ -8,7 +8,7 @@
     "flynn": "dist/cli/index.js"
   },
   "scripts": {
-    "build": "tsc",
+    "build": "tsc && cp -r src/gateway/ui dist/gateway/ui",
     "dev": "tsx watch src/cli/index.ts -- start",
     "start": "node dist/cli/index.js start",
     "tui": "node --no-deprecation --import tsx/esm src/cli/index.ts tui",