fix(reservations): clear editingReservation after successful save

When a reservation was saved, only setShowReservationModal(false) was
called. The modal's useEffect watches [reservation, isOpen, ...], so
flipping isOpen to false re-ran the effect with the stale editingReservation
(old assignment_id), resetting the form to the pre-edit state during the
closing animation. Users perceived this as the value reverting after save.

Calling setEditingReservation(null) immediately after the close mirrors
the existing onClose handler and prevents the stale-prop form reset.

client/src/pages/TripPlannerPage.tsx

@@ -642,6 +642,7 @@ export default function TripPlannerPage(): React.ReactElement | null {
         const r = await tripActions.updateReservation(tripId, editingReservation.id, { ...data, day_id: selectedDayId || null })
         toast.success(t('trip.toast.reservationUpdated'))
         setShowReservationModal(false)
+        setEditingReservation(null)
         if (data.type === 'hotel') {
           accommodationsApi.list(tripId).then(d => setTripAccommodations(d.accommodations || [])).catch(() => {})
         }