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/components/professional-calendar.tsx

@@ -613,7 +613,7 @@ function EventCreatorDialog({
               id="title"
               value={formData.title}
               onChange={(e) => setFormData({ ...formData, title: e.target.value })}
-              placeholder="Event title"
+              placeholder="Ereignistitel"
             />
           </div>
 
@@ -635,7 +635,7 @@ function EventCreatorDialog({
             </div>
 
             <div>
-              <Label htmlFor="date">Date</Label>
+              <Label htmlFor="date">Datum</Label>
               <Input
                 id="date"
                 type="date"
@@ -685,7 +685,7 @@ function EventCreatorDialog({
               id="location"
               value={formData.location}
               onChange={(e) => setFormData({ ...formData, location: e.target.value })}
-              placeholder="Event location"
+              placeholder="Ereignisort"
             />
           </div>
 
@@ -695,7 +695,7 @@ function EventCreatorDialog({
               id="description"
               value={formData.description}
               onChange={(e) => setFormData({ ...formData, description: e.target.value })}
-              placeholder="Event description"
+              placeholder="Ereignisbeschreibung"
               rows={3}
             />
           </div>