feat: display batch evaluation error details in UI

- Shows first 10 errors with activity_id and error message
- Helps admin debug evaluation failures
- Errors shown in error box with details

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/AdminTrainingProfiles.jsx

@@ -82,10 +82,22 @@ export default function AdminTrainingProfiles() {
 
     try {
       const result = await api.batchEvaluateActivities()
-      setSuccess(
-        `Batch-Evaluation abgeschlossen: ${result.stats.evaluated} evaluiert, ` +
+
+      let message = `Batch-Evaluation abgeschlossen: ${result.stats.evaluated} evaluiert, ` +
         `${result.stats.skipped} übersprungen, ${result.stats.errors} Fehler`
-      )
+
+      // Show error details if available
+      if (result.stats.error_details && result.stats.error_details.length > 0) {
+        message += '\n\nErste Fehler:\n' + result.stats.error_details.map(err =>
+          `- Aktivität ${err.activity_id} (Typ: ${err.training_type_id || 'keine'}): ${err.error}`
+        ).join('\n')
+      }
+
+      if (result.stats.errors > 0) {
+        setError(message)
+      } else {
+        setSuccess(message)
+      }
     } catch (e) {
       setError(e.message)
     }