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/app/(main)/api/auth/signup/route.ts

@@ -1,4 +1,5 @@
 import { NextRequest, NextResponse } from 'next/server';
+import { wwwUrl } from '@/lib/hosts';
 import bcrypt from 'bcryptjs';
 import crypto from 'crypto';
 import { db } from '@/lib/db';
@@ -17,7 +18,9 @@ import { triggerLifecycleScoring } from '@/lib/revops-server';
 
 async function issueVerificationEmail(user: { email: string; name: string | null }) {
   const verificationToken = crypto.randomBytes(32).toString('base64url');
-  const verificationUrl = new URL('/api/auth/verify-email', process.env.NEXT_PUBLIC_APP_URL || 'https://www.qrmaster.net');
+  // Public link in an outgoing email, so it points at the marketing host. The endpoint
+  // itself is served on both hosts and redirects into the app afterwards.
+  const verificationUrl = new URL(wwwUrl('/api/auth/verify-email'));
   verificationUrl.searchParams.set('token', verificationToken);
 
   await db.verificationToken.deleteMany({ where: { identifier: user.email } });
@@ -147,7 +150,7 @@ export async function POST(request: NextRequest) {
         fbc: request.cookies.get('_fbc')?.value,
         fbp: request.cookies.get('_fbp')?.value,
       },
-      eventSourceUrl: `${process.env.NEXT_PUBLIC_APP_URL}/signup`,
+      eventSourceUrl: wwwUrl('/signup'),
     }).catch(console.error);
 
     // Create response