revert(mf): drop microfrontends — VNCmail+ is a standalone project

Remove withMicrofrontends wrap + @vercel/microfrontends dep. Grouping is
organizational (separate Vercel team), not a microfrontends group. App
serves at its own root again (NEXT_PUBLIC_BASE_PATH removed on Vercel).

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

package.json

@@ -51,7 +51,6 @@
     "@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",