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/admin/WalkInCreateModal.svelte

@@ -5,6 +5,7 @@
 	import { SvelteDate } from 'svelte/reactivity';
 	import { getLocalTimeZone } from '@internationalized/date';
 	import { isValidUKPhone, toE164UK } from '$lib/utils/phone';
+	import { formatUserName } from '$lib/utils/nameDisplay';
 
 	// UI Components
 	import * as Modal from '$lib/components/ui/dialog';
@@ -50,7 +51,7 @@
 	let userType = $state<'member' | 'guest'>('member');
 	let userQuery = $state('');
 	let users = $state<
-		Array<{ id: string; full_name: string; email?: string; phone?: string; account_role: string }>
+		Array<{ id: string; full_name: string; email?: string; phone?: string; account_role: string; previous_first_name?: string | null; previous_last_name?: string | null }>
 	>([]);
 	let selectedUserId = $state<string | null>(null);
 	let guestName = $state('');
@@ -717,7 +718,7 @@
 													onclick={() => (selectedUserId = user.id)}
 												>
 													<div>
-														<div class="text-base font-medium">{user.full_name}</div>
+														<div class="text-base font-medium">{formatUserName(user.full_name, user.previous_first_name, user.previous_last_name)}</div>
 														<div class="text-xs text-gray-500">
 															{#if user.email && user.phone}
 																{user.email} • {user.phone}