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/routes/admin/schedule/+page.svelte

@@ -8,6 +8,7 @@
 	import { Skeleton } from '$lib/components/ui/skeleton';
 	import { Button } from '$lib/components/ui/button';
 	import { formatDuration } from '$lib/utils/format';
+	import { formatUserName } from '$lib/utils/nameDisplay';
 	import { SvelteDate } from 'svelte/reactivity';
 	import BookingModal from '$lib/components/admin/BookingModal.svelte';
 
@@ -18,7 +19,7 @@
 		start_time: string;
 		status: string;
 		duration_minutes: number;
-		user?: { full_name: string };
+		user?: { full_name: string; previous_first_name?: string | null; previous_last_name?: string | null };
 		services: BookingService[];
 	};
 	type TimeBlocker = {
@@ -554,7 +555,7 @@
 													{#if !hasOverlap}
 														<button
 															type="button"
-															aria-label="View booking for {b.user?.full_name || 'Guest'}"
+															aria-label="View booking for {formatUserName(b.user?.full_name || 'Guest', b.user?.previous_first_name, b.user?.previous_last_name)}"
 															class="absolute inset-x-0.5 z-10 cursor-pointer overflow-hidden rounded border px-1.5 py-0.5 text-left text-xs transition-shadow hover:shadow-md focus-visible:outline-2 focus-visible:outline-offset-1 focus-visible:outline-blue-500"
 															style="top: {topOffset}px; height: {heightPx}px; {bookingStyle(
 																b.status
@@ -570,7 +571,7 @@
 																	style="background:{dotColor(b.status)}"
 																></div>
 																<span class="truncate font-medium"
-																	>{b.user?.full_name || 'Guest'}</span
+																	>{formatUserName(b.user?.full_name || 'Guest', b.user?.previous_first_name, b.user?.previous_last_name)}</span
 																>
 															</div>
 															{#if heightPx > 36}