i18n: Fix 18 more hardcoded English strings in admin pages and calendar

Fixed files:
- admin/compliance-overview/page.tsx: 2 strings (error message, consultations)
- admin/exam-proctoring/page.tsx: 1 string (Alerts)
- admin/signage/page.tsx: 15 strings (tabs, labels, options, buttons)
- professional-calendar.tsx: 4 strings (event title, date, location, description)

All admin pages and calendar components now fully German.

src/app/admin/exam-proctoring/page.tsx

@@ -131,7 +131,7 @@ export default function ExamProctoringPage() {
         <div className="bg-white rounded-lg border border-gray-200 p-4">
           <div className="flex items-center justify-between">
             <div>
-              <p className="text-gray-600 text-sm font-medium">Alerts</p>
+              <p className="text-gray-600 text-sm font-medium">Warnungen</p>
               <p className="text-2xl font-bold text-gray-900 mt-1">{alertCount}</p>
             </div>
             <AlertCircle className={`w-8 h-8 ${alertCount > 0 ? 'text-red-500' : 'text-green-500'}`} />