fix: refine unavailable slot grouping validation logic

frontend/src/lib/components/booking/BookingFlow.svelte

@@ -554,15 +554,16 @@
 
 			if (isAvailable) {
 				if (currentUnavailableStart !== null) {
-					// Use the end time of the last available slot as the start of unavailable period
-					const unavailableStartTime = lastAvailableEndTime || currentUnavailableStart;
 					const groupEndTime = calculatePreviousTime(timeStr);
-					groupedSlots.push({
+					const unavailableStartTime = currentUnavailableStart || lastAvailableEndTime;
-						type: 'unavailable',
+					if (unavailableStartTime && timeToMinutes(unavailableStartTime) < timeToMinutes(groupEndTime)) {
-						startTime: unavailableStartTime,
+						groupedSlots.push({
-						endTime: groupEndTime,
+							type: 'unavailable',
-						isGrouped: true
+							startTime: unavailableStartTime,
-					});
+							endTime: groupEndTime,
+							isGrouped: true
+						});
+					}
 					currentUnavailableStart = null;
 				}