Fix: rental booking save (date comparison), add DB guest search in rental modal

- BookingCalendar: normalize date comparison with .slice(0,10) to fix PostgreSQL DATE ISO format mismatch
- CalendarPage: show alert on rental booking save failure
- RentalBookingModal: add debounced DB guest search (api.guests.list) with 300ms delay
- RentalBookingModal: combine booking suggestions + DB results in unified dropdown
- Pass slug prop to RentalBookingModal for API access

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/calendar/BookingCalendar.tsx

@@ -669,7 +669,7 @@ export function BookingCalendar({ rooms, bookings, slug, onBookingCreate, onBook
                       const isWe = date.getDay() === 0 || date.getDay() === 6
                       const isTod = isToday(date)
                       const dayBookings = (rentalBookings ?? []).filter(
-                        b => b.objectId === obj.id && b.date === dateStr && b.status !== 'cancelled',
+                        b => b.objectId === obj.id && (b.date as string).slice(0, 10) === dateStr && b.status !== 'cancelled',
                       )
                       const hasFullDay = dayBookings.some(b => b.isFullDay)
 
@@ -791,10 +791,11 @@ export function BookingCalendar({ rooms, bookings, slug, onBookingCreate, onBook
           obj={rentalModal.obj}
           date={rentalModal.date}
           existingBookings={(rentalBookings ?? []).filter(
-            b => b.objectId === rentalModal.obj.id && b.date === rentalModal.date,
+            b => b.objectId === rentalModal.obj.id && (b.date as string).slice(0, 10) === rentalModal.date,
           )}
           bookings={bookings}
           rooms={rooms}
+          slug={slug}
           onClose={() => setRentalModal(null)}
           onSave={(b) => {
             onRentalBookingCreate?.(b)