feat(mobile): swap admin page headers to PageHeader

Mechanical sweep replacing the plain h1+p header markup with the
mobile-aware PageHeader primitive across 12 admin pages: index,
backup, branding, documenso, email, import, invitations, monitoring,
onboarding, reminders, reports, webhooks. Webhooks "Add Webhook"
button preserved via the actions slot.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/(dashboard)/[portSlug]/admin/invitations/page.tsx

@@ -1,15 +1,13 @@
 import { InvitationsManager } from '@/components/admin/invitations/invitations-manager';
+import { PageHeader } from '@/components/shared/page-header';
 
 export default function InvitationsPage() {
   return (
     <div className="space-y-6">
-      <div>
-        <h1 className="text-2xl font-semibold">Invitations</h1>
-        <p className="text-sm text-muted-foreground">
-          Send a single-use invitation to a new CRM user. The recipient sets their own password via
-          the link in the email.
-        </p>
-      </div>
+      <PageHeader
+        title="Invitations"
+        description="Send a single-use invitation to a new CRM user. The recipient sets their own password via the link in the email."
+      />
       <InvitationsManager />
     </div>
   );