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/page.tsx

@@ -20,6 +20,7 @@ import {
 } from 'lucide-react';
 
 import { Card, CardContent, CardDescription, CardHeader, CardTitle } from '@/components/ui/card';
+import { PageHeader } from '@/components/shared/page-header';
 
 interface AdminSection {
   href: string;
@@ -165,13 +166,10 @@ export default async function AdminLandingPage({
   const { portSlug } = await params;
   return (
     <div className="space-y-6">
-      <div>
-        <h1 className="text-2xl font-semibold">Administration</h1>
-        <p className="text-sm text-muted-foreground">
-          Per-port configuration and system administration. Each card below opens a dedicated
-          settings page.
-        </p>
-      </div>
+      <PageHeader
+        title="Administration"
+        description="Per-port configuration and system administration. Each card below opens a dedicated settings page."
+      />
       <div className="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 gap-4">
         {SECTIONS.map((s) => {
           const Icon = s.icon;