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/lib/components/today/CurrentAppointment.svelte

@@ -9,10 +9,11 @@
 
 	interface Props {
 		openBookingModal: (bookingId: string) => void;
+		openEditBookingModal: (bookingId: string, nextAppointmentStart?: string | null) => void;
 		openUserModal: (userId: string) => void;
 	}
 
-	let { openBookingModal, openUserModal }: Props = $props();
+	let { openBookingModal, openEditBookingModal, openUserModal }: Props = $props();
 
 	type Booking = {
 		id: string;
@@ -153,7 +154,7 @@
 
 	function handleEdit() {
 		if (currentAppointment) {
-			openBookingModal(currentAppointment.id);
+			openEditBookingModal(currentAppointment.id, nextAppointment?.start_time ?? null);
 		}
 	}