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/email/page.tsx

@@ -2,6 +2,7 @@ import {
   SettingsFormCard,
   type SettingFieldDef,
 } from '@/components/admin/shared/settings-form-card';
+import { PageHeader } from '@/components/shared/page-header';
 
 const FIELDS: SettingFieldDef[] = [
   {
@@ -79,13 +80,10 @@ const FIELDS: SettingFieldDef[] = [
 export default function EmailSettingsPage() {
   return (
     <div className="space-y-6">
-      <div>
-        <h1 className="text-2xl font-semibold">Email Settings</h1>
-        <p className="text-sm text-muted-foreground">
-          Per-port outgoing email configuration. SMTP credentials and the From address default to
-          environment variables when these fields are blank.
-        </p>
-      </div>
+      <PageHeader
+        title="Email Settings"
+        description="Per-port outgoing email configuration. SMTP credentials and the From address default to environment variables when these fields are blank."
+      />
       <SettingsFormCard
         title="From address & signature"
         description="Identity headers and shared HTML used by system-generated emails."