feat: add booking edit modal for /today page with service management

Replace placeholder BookingModal on the Next Appointment edit button with a
dedicated EditBookingModal that allows admins to add, remove, and override
services on an active booking. Includes backend PUT endpoint with overlap
detection and full test suite (20 tests).

frontend/src/routes/today/+page.svelte

@@ -12,6 +12,7 @@
 	import CallInBooking from '$lib/components/admin/CallInBooking.svelte';
 	import WalkInBooking from '$lib/components/admin/WalkInBooking.svelte';
 	import BookingModal from '$lib/components/admin/BookingModal.svelte';
+	import EditBookingModal from '$lib/components/admin/EditBookingModal.svelte';
 	import UserModal from '$lib/components/admin/UserModal.svelte';
 
 	// =============== Auth & Permissions ===============
@@ -42,15 +43,23 @@
 
 	// =============== Modal State ===============
 	let showBookingModal = $state(false);
+	let showEditBookingModal = $state(false);
 	let showUserModal = $state(false);
 	let selectedBookingId = $state<string | null>(null);
 	let selectedUserId = $state<string | null>(null);
+	let editBookingNextStart = $state<string | null>(null);
 
 	function openBookingModal(bookingId: string) {
 		selectedBookingId = bookingId;
 		showBookingModal = true;
 	}
 
+	function openEditBookingModal(bookingId: string, nextAppointmentStart: string | null = null) {
+		selectedBookingId = bookingId;
+		editBookingNextStart = nextAppointmentStart;
+		showEditBookingModal = true;
+	}
+
 	function openUserModal(userId: string) {
 		selectedUserId = userId;
 		showUserModal = true;
@@ -104,7 +113,7 @@
 		</div>
 
 		<!-- Current/Next Appointment Card (Full Width) -->
-		<CurrentAppointment {openBookingModal} {openUserModal} />
+		<CurrentAppointment {openBookingModal} {openEditBookingModal} {openUserModal} />
 
 		<!-- quick booking Grid -->
 		<div class="grid grid-cols-1 gap-6 lg:grid-cols-3">
@@ -148,6 +157,18 @@
 		<BookingModal bind:open={showBookingModal} bookingId={selectedBookingId} />
 	{/if}
 
+	{#if showEditBookingModal && selectedBookingId}
+		<EditBookingModal
+			bind:open={showEditBookingModal}
+			bookingId={selectedBookingId}
+			nextAppointmentStart={editBookingNextStart}
+			onSaved={() => {
+				// Refresh the page data after save
+				showBookingModal = false;
+			}}
+		/>
+	{/if}
+
 	{#if showUserModal && selectedUserId}
 		<UserModal bind:open={showUserModal} userId={selectedUserId} {openBookingModal} />
 	{/if}