feat(frontend): add former name display, referral discount type, and refunds to bookings

Update frontend types, stores, and components to support name history display and new API fields.

- Add previousFirstName/previousLastName to User type and BookingUser type
- Add referral discount_source type and refunds array to Booking type
- Create nameDisplay.ts utility for rendering '(formerly ...)' labels
- Create booking.ts utility for booking-related helpers
- Update 25+ admin, payments, today, and account components to display
  former names on booking cards, modals, appointment views, and user lists

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

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

frontend/src/lib/components/today/CurrentAppointment.svelte

@@ -3,6 +3,7 @@
 	import { SvelteDate } from 'svelte/reactivity';
 	import { toast } from 'svelte-sonner';
 	import { formatDuration } from '$lib/utils/format';
+	import { formatUserName } from '$lib/utils/nameDisplay';
 	import * as Card from '$lib/components/ui/card';
 	import { Button } from '$lib/components/ui/button';
 	import { Badge } from '$lib/components/ui/badge';
@@ -35,6 +36,8 @@
 			full_name: string;
 			phone?: string;
 			profile_pic_url?: string;
+			previous_first_name?: string;
+			previous_last_name?: string;
 		};
 		services: Array<{
 			service_name?: string;
@@ -645,7 +648,7 @@
 					{#if activeAppointment.user?.profile_pic_url}
 						<img
 							src={activeAppointment.user.profile_pic_url}
-							alt={activeAppointment.user.full_name}
+							alt={formatUserName(activeAppointment.user.full_name, activeAppointment.user.previous_first_name, activeAppointment.user.previous_last_name)}
 							class="h-14 w-14 shrink-0 rounded-full object-cover ring-2 ring-blue-200 sm:h-16 sm:w-16 sm:ring-4 md:h-20 md:w-20"
 						/>
 					{:else}
@@ -660,11 +663,11 @@
 							{initials}
 						</div>
 					{/if}
-					<div class="min-w-0">
-						<div class="truncate text-lg font-semibold">
-							{activeAppointment.user?.full_name || 'Guest'}
-						</div>
-						<div class="truncate text-sm text-gray-600">{activeAppointment.user?.phone || '—'}</div>
+				<div class="min-w-0">
+					<div class="truncate text-lg font-semibold">
+						{formatUserName(activeAppointment.user?.full_name || 'Guest', activeAppointment.user?.previous_first_name, activeAppointment.user?.previous_last_name)}
+					</div>
+					<div class="truncate text-sm text-gray-600">{activeAppointment.user?.phone || '—'}</div>
 						{#if activeAppointment.user}
 							<button
 								type="button"