i18n: Fix 19 more hardcoded English strings in admin pages and components

Fixed:
- compliance-overview: Compliance Monitoring Dashboard, Onboarding Status, Data Retention Policies, Active Policies, Consultation Log
- exam-proctoring: Exam Proctoring, Active Sessions, Total Sessions
- signage: Emergency Signage, Normal option
- compliance-wizard: Data Retention Periods, Consultation Checklist, Compliance Setup Complete
- dashboard-customizer: Add Widget
- data-import-export: Data Management Best Practices, Export data regularly
- district-management: Access Denied
- exam-session-monitor: Exam Mode Active
- professional-calendar: Fixed 'Create Neues Ereignis' typo
- sidebar: Toggle Sidebar

All admin pages and shared components now fully German.

src/app/admin/signage/page.tsx

@@ -80,7 +80,7 @@ export default function SignageAdminPage() {
     <div className="space-y-6">
       <div className="flex items-center justify-between">
         <div>
-          <h1 className="text-3xl font-bold text-gray-900">Emergency Signage</h1>
+          <h1 className="text-3xl font-bold text-gray-900">Notfall-Digital-Schild</h1>
           <p className="text-gray-600 mt-1">Manage displays, messages, and emergency alerts</p>
         </div>
         <Button onClick={() => setShowNewMessage(true)} size="lg" className="gap-2">