feat: unify walk-in and call-in reservation flows with 15min TTL, guest booking support, and slot awareness

- backend/handlers/bookings/admin_reserve.go:
  - Add explicit reservation_type field ("walkin" | "callin") to request struct
  - Remove TTL-based heuristic for type detection
  - Walk-in: uses duration_minutes, allows null user_id, 1min past grace
  - Call-in: requires service_ids, validates future time, calculates duration from services
  - Both types now use 15-minute TTL

- backend/handlers/scheduling/time-blockers.go:
  - Update CleanupOldReservations: both walkin and callin use 15min TTL (was 10min/60min)

- frontend/WalkInBooking.svelte:
  - Full rewrite of reservation logic
  - If available now and >15min remaining: reserve from now to slot end
  - If <=15min or not available: reserve next full slot
  - Always reserves before opening modal (never open without hold)
  - Passes reservedDuration to modal
  - TTL changed from 5 to 15 minutes

- frontend/WalkInCreateModal.svelte:
  - Replace dead commented-out guest code with working guest creation
  - Guest account created at submit time (not earlier)
  - Phone defaults to +447700900000 if blank
  - Phone field marked optional with helper text
  - Name split into firstName/lastName for backend
  - Validation relaxed: only name required for guests

- frontend/BookingCreateModal.svelte:
  - TTL changed from 60 to 15 minutes
  - Add reservation_type: "callin" to reserve payload
  - Guest creation uses correct firstName/lastName fields
  - Default guest phone to +447700900000
  - Reservation no longer requires selectedUserId (works for guests)

- docs: Update Future Work backlog to mark completed items

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

@@ -361,7 +361,7 @@
 
 	// =============== Reservation ===============
 	async function reserveSlot(): Promise<boolean> {
-		if (!selectedUserId || !selectedDate || !selectedTime) {
+		if (!selectedDate || !selectedTime) {
 			return false;
 		}
 
@@ -387,19 +387,22 @@
 				}
 			}
 
+			const payload = {
+				user_id: selectedUserId || null,
+				start_time: startTimeISO,
+				service_ids: serviceIds,
+				service_overrides: overrides.length > 0 ? overrides : [],
+				ttl_minutes: 15,
+				reservation_type: 'callin'
+			};
+
 			const response = await fetch('/api/admin/bookings/reserve', {
 				method: 'POST',
 				headers: {
 					'Content-Type': 'application/json',
 					Authorization: `Bearer ${authStore.currentToken}`
 				},
-				body: JSON.stringify({
-					user_id: selectedUserId,
-					start_time: startTimeISO,
-					service_ids: serviceIds,
-					service_overrides: overrides.length > 0 ? overrides : [],
-					ttl_minutes: 60
-				})
+				body: JSON.stringify(payload)
 			});
 
 			if (response.ok) {
@@ -732,28 +735,32 @@
 		try {
 			let finalUserId = selectedUserId;
 
-			if (userType === 'guest') {
-				const createRes = await fetch('/api/users/guest', {
-					method: 'POST',
-					headers: {
-						'Content-Type': 'application/json',
-						Authorization: `Bearer ${authStore.currentToken}`
-					},
-					body: JSON.stringify({
-						name: guestName,
-						phone: guestPhone
-					})
-				});
+		if (userType === 'guest') {
+			const phone = guestPhone.trim() || '+447700900000';
+			const createRes = await fetch('/api/users/guest', {
+				method: 'POST',
+				headers: {
+					'Content-Type': 'application/json',
+					Authorization: `Bearer ${authStore.currentToken}`
+				},
+				body: JSON.stringify({
+					firstName: guestName.trim().split(' ')[0] || 'Guest',
+					lastName: guestName.trim().split(' ').slice(1).join(' ') || 'Customer',
+					phone: phone,
+					email: `callin-${Date.now()}@guest.invalid`
+				})
+			});
 
-				if (!createRes.ok) {
-					toast.error('Failed to create guest user');
-					return;
-				}
-
-				const guestUser = await createRes.json();
-				finalUserId = guestUser.id;
+			if (!createRes.ok) {
+				toast.error('Failed to create guest user');
+				submitting = false;
+				return;
 			}
 
+			const guestUser = await createRes.json();
+			finalUserId = guestUser.id;
+		}
+
 			if (!finalUserId) throw new Error('User ID required');
 			if (!selectedDate || !selectedTime) throw new Error('Date and time required');