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>

src/components/dashboard/QRCodeCard.tsx

@@ -7,6 +7,7 @@ import { Card, CardContent } from '@/components/ui/Card';
 import { Badge } from '@/components/ui/Badge';
 import { Dropdown, DropdownItem } from '@/components/ui/Dropdown';
 import { formatDate } from '@/lib/utils';
+import { getWwwOrigin } from '@/lib/hosts';
 import {
   ONBOARDING_DOWNLOAD_COMPLETE_EVENT,
   ONBOARDING_DOWNLOAD_COMPLETE_KEY,
@@ -79,7 +80,10 @@ export const QRCodeCard: React.FC<QRCodeCardProps> = ({
 
   // For dynamic QR codes, use the redirect URL for tracking
   // For static QR codes, use the direct URL from content
-  const baseUrl = process.env.NEXT_PUBLIC_APP_URL || (typeof window !== 'undefined' ? window.location.origin : 'http://localhost:3050');
+  //
+  // Must be the WWW origin, never the app origin: this value gets encoded into the QR
+  // code the user downloads and prints. /r/<slug> is served by the marketing host.
+  const baseUrl = getWwwOrigin();
 
   // Get the QR URL based on type
   let qrUrl = '';