fix: canonicalize marketing routes on writehumanly.net

[ShenzheZhu]

Closes #1019

Summary

• canonicalize public marketing routes on https://writehumanly.net
• redirect the old https://app.writehumanly.net/ homepage and marketing pages back to the apex domain
• keep authenticated/product routes on https://app.writehumanly.net
• preserve query strings and exempt infrastructure/static asset requests
• update the About availability section with the managed SaaS and self-hosted deployment positioning
• add production deploy assertions for both redirect directions

Route ownership

Surface	Canonical host
/, /about, /research, /docs, /privacy, /terms	writehumanly.net
/login, /register, /documents, /certificates, /verify	app.writehumanly.net

Verification

• pnpm --filter @humanly/frontend-user type-check
• pnpm build:all
• pnpm build:frontend-user after the final root-route fix
• local production host-header smoke:
  • apex homepage/About/Research/Docs: 200
  • apex /documents?view=grid: 308 to app host with query preserved
  • app /: 308 to apex homepage
  • app /about?ref=nav: 308 to apex About with query preserved
  • app /login and /documents: 200

Notes

The deploy workflow now fails if these canonical host contracts regress.