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)/r/[slug]/route.ts

@@ -1,4 +1,7 @@
 import { NextRequest, NextResponse } from 'next/server';
+// These landing pages (/vcard, /display, /coupon, /feedback) are public marketing-host
+// pages reached straight from a scanned QR code - never the app host.
+import { getWwwOrigin } from '@/lib/hosts';
 import { db } from '@/lib/db';
 import { hashIP } from '@/lib/hash';
 import { triggerLifecycleScoring } from '@/lib/revops-server';
@@ -47,7 +50,7 @@ export async function GET(
         break;
       case 'VCARD':
         // For vCard, redirect to display page
-        const baseUrlVcard = process.env.NEXT_PUBLIC_APP_URL || 'http://localhost:3050';
+        const baseUrlVcard = getWwwOrigin();
         destination = `${baseUrlVcard}/vcard?firstName=${encodeURIComponent(content.firstName || '')}&lastName=${encodeURIComponent(content.lastName || '')}&email=${encodeURIComponent(content.email || '')}&phone=${encodeURIComponent(content.phone || '')}&organization=${encodeURIComponent(content.organization || '')}&title=${encodeURIComponent(content.title || '')}`;
         break;
       case 'GEO':
@@ -58,7 +61,7 @@ export async function GET(
         break;
       case 'TEXT':
         // For plain text, redirect to a display page
-        const baseUrlText = process.env.NEXT_PUBLIC_APP_URL || 'http://localhost:3050';
+        const baseUrlText = getWwwOrigin();
         destination = `${baseUrlText}/display?text=${encodeURIComponent(content.text || '')}`;
         break;
       case 'PDF':
@@ -81,12 +84,12 @@ export async function GET(
         break;
       case 'COUPON':
         // Redirect to coupon display page
-        const baseUrlCoupon = process.env.NEXT_PUBLIC_APP_URL || 'http://localhost:3050';
+        const baseUrlCoupon = getWwwOrigin();
         destination = `${baseUrlCoupon}/coupon/${slug}`;
         break;
       case 'FEEDBACK':
         // Redirect to feedback form page
-        const baseUrlFeedback = process.env.NEXT_PUBLIC_APP_URL || 'http://localhost:3050';
+        const baseUrlFeedback = getWwwOrigin();
         destination = `${baseUrlFeedback}/feedback/${slug}`;
         break;
       case 'BARCODE':