Serve the app on app.qrmaster.net, marketing on www

Splits the two hostnames across one deployment. No files move: the Next app still
serves every route on both hosts, and the middleware decides per host which paths it
owns and 301s the rest. /login and /signup stay on www - all 82 marketing CTAs point
at /signup, which carries a hard canonical to www plus ad traffic.

src/lib/hosts.ts is the single source of truth for the boundary (APP_PATH_PREFIXES,
isAppPath, wwwUrl, appUrl, urlForPath). The middleware and every absolute-URL builder
read from it so they cannot drift apart.

- Split the overloaded NEXT_PUBLIC_APP_URL into a www and an app origin. It previously
  fed both public URLs and in-app URLs, so any single value was wrong somewhere. Most
  important: QRCodeCard encodes this origin into the QR code the user downloads and
  prints, so it must stay on www.
- Route Stripe return URLs, email links and OAuth redirects per path rather than
  against one origin, so /dashboard lands on app and /pricing on www.
- Cross the host boundary once, after a successful login: the router cannot push across
  origins, so that jump needs a full load. The user arrives signed in because the
  session cookie is scoped to COOKIE_DOMAIN.
- Keep the app host out of search indexes: X-Robots-Tag on every response plus a
  Disallow-all robots.txt via rewrite, and /sitemap.xml redirects to www.
- Point the TikTok callback fallback at www explicitly. It used to read
  NEXT_PUBLIC_APP_URL, whose meaning changed here, and only the apex domain is
  verified with TikTok.

Host splitting is inert while both origins are equal, so development is unaffected.

Verified: tsc clean, production build succeeds including the Edge middleware bundle,
and the path-to-host mapping is unit-checked (prefix traps like /created and
/settings-guide stay on www, query strings do not break matching).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Dockerfile

@@ -31,7 +31,10 @@ ENV NEXTAUTH_SECRET="build-time-secret"
 ENV IP_SALT="build-time-salt"
 ENV STRIPE_SECRET_KEY="sk_test_placeholder_for_build"
 ENV RESEND_API_KEY="re_placeholder_for_build"
-ENV NEXT_PUBLIC_APP_URL="https://www.qrmaster.net"
+# Marketing host vs app host. NEXT_PUBLIC_WWW_URL must stay on www: it is the origin
+# encoded into downloaded QR codes and used for public email links.
+ENV NEXT_PUBLIC_WWW_URL="https://www.qrmaster.net"
+ENV NEXT_PUBLIC_APP_URL="https://app.qrmaster.net"
 # PostHog Analytics - REQUIRED at build time for client-side bundle
 ENV NEXT_PUBLIC_POSTHOG_KEY="phc_97JBJVVQlqqiZuTVRHuBnnG9HasOv3GSsdeVjossizJ"
 ENV NEXT_PUBLIC_POSTHOG_HOST="https://us.i.posthog.com"