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/lib/api.ts

@@ -47,9 +47,10 @@ export interface DatasetProperties {
 export interface Snapshot {
   name: string
   dataset?: string
-  creation: number
+  created: number
   used: number
   referenced: number
+  creation_datetime?: string
 }
 
 export interface SambaShare {