feat(site): landing-only on www, app gated to demo subdomain (#20)

* feat(site): landing-only on www host, app gated to demo subdomain

- RequireAppHost redirects app routes to / on non-demo hosts
- Signup CTAs swapped for launch-notification mailto on www
- Pricing marked as indicative pre-launch, no subscription purchasable
- SEO: OG/Twitter cards, og.png, JSON-LD, robots.txt, sitemap.xml,
  per-route document titles, SPA rewrite for Vercel

Authored By: TDvorak <[email protected]>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* chore: ignore vercel link artifacts

Authored By: TDvorak <[email protected]>

* docs: document www/demo host split in deployment targets

Authored By: TDvorak <[email protected]>

---------

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/frontend/public/robots.txt

@@ -0,0 +1,8 @@
+User-agent: *
+Allow: /
+Disallow: /dashboard
+Disallow: /book/
+Disallow: /manage/
+Disallow: /auth/
+
+Sitemap: https://bookra.eu/sitemap.xml