fix: port slot grouping fixes across all booking journeys

UserBookingModal (reschedule): add lunch protection filtering, validation guard preventing invalid start>end unavailable blocks, composite each keys, and simplified available/unavailable rendering (lunch is invisible to users).

BookingCreateModal (admin): add validation guard for unavailable blocks and fix composite each keys. Admin lunch protection behavior (30min min + amber warning) preserved.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

frontend/src/lib/components/admin/BookingCreateModal.svelte

@@ -652,12 +652,14 @@
 				if (currentUnavailableStart !== null) {
 					const unavailableStartTime = lastAvailableEndTime || currentUnavailableStart;
 					const groupEndTime = calculatePreviousTime(timeStr);
-					groupedSlots.push({
-						type: 'unavailable',
-						startTime: unavailableStartTime,
-						endTime: groupEndTime,
-						isGrouped: true
-					});
+					if (timeToMinutes(unavailableStartTime) < timeToMinutes(groupEndTime)) {
+						groupedSlots.push({
+							type: 'unavailable',
+							startTime: unavailableStartTime,
+							endTime: groupEndTime,
+							isGrouped: true
+						});
+					}
 					currentUnavailableStart = null;
 				}
 
@@ -1263,7 +1265,7 @@
 
 								{#if groupedTimeSlots.length > 0}
 									<div class="grid gap-2">
-										{#each groupedTimeSlots as slot (slot.startTime)}
+										{#each groupedTimeSlots as slot (slot.type + '-' + slot.startTime + '-' + slot.endTime)}
 											{#if slot.type === 'available'}
 												{@const protection = lunchProtectionStatus().get(slot.startTime)}
 												{#if protection?.isBlocked}