Fix: TypeScript-Fehler für Multi-Treatment-Migration beheben

- admin-calendar.tsx: getTreatmentNames für treatments[] angepasst
- admin-calendar.tsx: getAvailableTimes für treatmentIds[] umgestellt
- admin-calendar.tsx: createManualBooking sendet treatments[] statt treatmentId
- cancellation.ts: treatmentId optional behandeln (Rückwärtskompatibilität)
- review-submission-page.tsx: treatmentName durch treatments[] ersetzt
- booking-status-page.tsx: proposed date/time als optional markiert

Docker-Build erfolgreich getestet.

src/client/components/booking-status-page.tsx

@@ -45,7 +45,7 @@ interface RescheduleProposalDetails {
     totalPrice: number;
   };
   original: { date: string; time: string };
-  proposed: { date: string; time: string };
+  proposed: { date?: string; time?: string };
   expiresAt: string;
   hoursUntilExpiry: number;
   isExpired: boolean;
@@ -203,7 +203,7 @@ export default function BookingStatusPage({ token }: BookingStatusPageProps) {
       
       if (oneClickAction === 'accept') {
         const confirmAccept = window.confirm(
-          `Möchtest du den neuen Termin am ${rescheduleProposal.proposed.date} um ${rescheduleProposal.proposed.time} Uhr akzeptieren?`
+          `Möchtest du den neuen Termin am ${rescheduleProposal.proposed.date || 'TBD'} um ${rescheduleProposal.proposed.time || 'TBD'} Uhr akzeptieren?`
         );
         if (confirmAccept) {
           acceptMutation.mutate({ token });
@@ -381,7 +381,7 @@ export default function BookingStatusPage({ token }: BookingStatusPageProps) {
               </div>
               <div className="border rounded-lg p-4 bg-orange-50">
                 <div className="text-sm text-orange-700 font-semibold mb-1">Neuer Vorschlag</div>
-                <div className="text-gray-900 font-medium">{rescheduleProposal.proposed.date} um {rescheduleProposal.proposed.time} Uhr</div>
+                <div className="text-gray-900 font-medium">{rescheduleProposal.proposed.date || 'TBD'} um {rescheduleProposal.proposed.time || 'TBD'} Uhr</div>
                 <div className="text-gray-700 text-sm mt-2">
                   {rescheduleProposal.booking.treatments && rescheduleProposal.booking.treatments.length > 0 ? (
                     <>