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/reminders/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 DEFAULT_FIELDS: SettingFieldDef[] = [
   {
@@ -53,14 +54,10 @@ const DIGEST_FIELDS: SettingFieldDef[] = [
 export default function ReminderSettingsPage() {
   return (
     <div className="space-y-6">
-      <div>
-        <h1 className="text-2xl font-semibold">Reminders</h1>
-        <p className="text-sm text-muted-foreground">
-          Default reminder behaviour for new interests and the optional daily-digest delivery
-          window. Individual users can still configure their own digest preferences in Notifications
-          → Preferences.
-        </p>
-      </div>
+      <PageHeader
+        title="Reminders"
+        description="Default reminder behaviour for new interests and the optional daily-digest delivery window. Individual users can still configure their own digest preferences in Notifications → Preferences."
+      />
 
       <SettingsFormCard
         title="Defaults for new interests"