Fix: Snapshot-Datum 'Invalid Date' - Feld created statt creation

Backend liefert das Snapshot-Erstellungsdatum als 'created' (Unix-Timestamp),
das Frontend-Interface/Rendering erwartete aber 'creation'. Feld angeglichen
und creation_datetime im Interface ergänzt.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/app/snapshots/page.tsx

@@ -190,7 +190,7 @@ export default function SnapshotsPage() {
               </div>
             </td>
             <td className="py-3 px-4 font-mono text-xs font-medium">{tag}</td>
-            <td className="py-3 px-4 text-muted-foreground">{formatUnix(snap.creation)}</td>
+            <td className="py-3 px-4 text-muted-foreground">{formatUnix(snap.created)}</td>
             <td className="py-3 px-4">{formatBytes(snap.used)}</td>
             <td className="py-3 px-4">{formatBytes(snap.referenced)}</td>
             <td className="py-3 px-4 text-right">