chore(cutover): the docs site moves to next 16.4.1, off three next advisories (bill item 40, objectstack#25803)

next 16.3.8 carries GHSA-2464-fhr4-w9xw and GHSA-673q-mjm6-r5xr (critical) and
GHSA-p3m5-59mp-jv5r (high), all fixed in 16.4.1. The fumadocs peers admit it (core and ui
16.15.4 ask next 16.x.x, mdx 15.2.3 ^15.3.0 || ^16.0.0). 16.4.1 rather than the latest 16.4.3
because objectstack's apps/docs is already on 16.4.1, so the two sites share one copy. The site
is private and excluded from the build filter.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CUXv64KLMfYv3ia2sWi6te

Author: claude

ui/apps/site/package.json

@@ -37,7 +37,7 @@
     "fumadocs-mdx": "15.2.3",
     "fumadocs-ui": "16.15.4",
     "lucide-react": "^1.43.0",
-    "next": "16.3.8",
+    "next": "16.4.1",
     "react": "19.3.0",
     "react-dom": "19.3.0",
     "tailwind-merge": "^3.6.0"