fix(reservations): prevent Invalid Date when end time is set without end date (#866)

When reservation_end_time held a bare time string ("HH:MM"), fmtDate()
produced Invalid Date on the reservation card.

- Modal: when end date is blank but end time is filled, construct a
  same-day ISO datetime using the start date (prevents time-only strings
  from ever being persisted)
- Panel: derive endDatePart via regex so date-only end values ("YYYY-MM-DD")
  still show the multi-day range, while bare time strings are skipped and
  handled correctly by the existing time column logic

Closes #860

client/src/components/Planner/ReservationModal.tsx

@@ -182,6 +182,8 @@ export function ReservationModal({ isOpen, onClose, onSave, reservation, days, p
       let combinedEndTime = form.reservation_end_time
       if (form.end_date) {
         combinedEndTime = form.reservation_end_time ? `${form.end_date}T${form.reservation_end_time}` : form.end_date
+      } else if (form.reservation_end_time && form.reservation_time) {
+        combinedEndTime = `${form.reservation_time.split('T')[0]}T${form.reservation_end_time}`
       }
       if (isBudgetEnabled) {
         if (form.price) metadata.price = form.price

client/src/components/Planner/ReservationsPanel.tsx

@@ -236,7 +236,16 @@ function ReservationCard({ r, tripId, onEdit, onDelete, files = [], onNavigateTo
               <div style={fieldLabelStyle}>{t('reservations.date')}</div>
               <div style={{ ...fieldValueStyle, textAlign: 'center' }}>
                 {fmtDate(r.reservation_time)}
-                {r.reservation_end_time && (r.reservation_end_time.includes('T') ? r.reservation_end_time.split('T')[0] : r.reservation_end_time) !== r.reservation_time.split('T')[0] && (
+                {(() => {
+                  const endDatePart = r.reservation_end_time
+                    ? r.reservation_end_time.includes('T')
+                        ? r.reservation_end_time.split('T')[0]
+                        : /^\d{4}-\d{2}-\d{2}$/.test(r.reservation_end_time)
+                            ? r.reservation_end_time
+                            : null
+                    : null
+                  return endDatePart && endDatePart !== r.reservation_time.split('T')[0]
+                })() && (
                   <> – {fmtDate(r.reservation_end_time)}</>
                 )}
               </div>