feat(mf): join VNClagoon Suite microfrontends group

Wrap next.config with withMicrofrontends; add @vercel/microfrontends.
Served at /mail under the suite shell (via NEXT_PUBLIC_BASE_PATH set on the
Vercel project). Logged in vnc/VNC-CHANGES.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -51,6 +51,7 @@
     "@tiptap/pm": "^3.22.4",
     "@tiptap/react": "^3.22.4",
     "@tiptap/starter-kit": "^3.22.4",
+    "@vercel/microfrontends": "^2.4.0",
     "asn1js": "^3.0.10",
     "clsx": "^2.1.1",
     "date-fns": "^4.1.0",