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/TimeBlockers.svelte

@@ -4,6 +4,7 @@
 	import { toast } from 'svelte-sonner';
 	import { sanitizeText } from '$lib/utils/toast-safe';
 	import { formatDuration } from '$lib/utils/format';
+	import { formatUserName } from '$lib/utils/nameDisplay';
 
 	import { Button } from '$lib/components/ui/button';
 	import * as Card from '$lib/components/ui/card';
@@ -33,6 +34,8 @@
 			full_name: string;
 			email: string | null;
 			phone: string | null;
+			previous_first_name?: string | null;
+			previous_last_name?: string | null;
 		} | null;
 		services: string[];
 	};
@@ -850,7 +853,7 @@
 						{#each overlappingBookings as booking (booking.id)}
 							<div class="rounded-md border border-amber-200 bg-white p-3">
 								<div class="min-w-0">
-									<div class="text-sm font-medium">{booking.user?.full_name || 'Unknown'}</div>
+									<div class="text-sm font-medium">{formatUserName(booking.user?.full_name || 'Unknown', booking.user?.previous_first_name, booking.user?.previous_last_name)}</div>
 									<div class="text-xs text-gray-500">
 										{new SvelteDate(booking.start_time).toLocaleTimeString('en-GB', {
 											hour: 'numeric',