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/payments/UserPaymentModal.svelte

@@ -220,7 +220,10 @@ import { SvelteDate } from 'svelte/reactivity';
 	let defaultType = $derived(
 		defaultPaymentType ?? (depositOutstanding ? 'deposit' : 'full')
 	);
-	let paymentType = $state<'full' | 'partial' | 'deposit'>(defaultType as 'full' | 'partial' | 'deposit');
+	let paymentType = $state<'full' | 'partial' | 'deposit'>('full');
+	$effect(() => {
+		paymentType = defaultType as 'full' | 'partial' | 'deposit';
+	});
 
 	let pollingInterval: ReturnType<typeof setInterval> | null = null;
 
@@ -923,6 +926,11 @@ import { SvelteDate } from 'svelte/reactivity';
 							disabled={false}
 						/>
 					{/if}
+
+					<!-- Card validation error shown inline near card input -->
+					{#if cardValidationError}
+						<p class="text-sm text-red-600">{cardValidationError}</p>
+					{/if}
 				{/if}
 
 				{#if depositPolicyWarning}
@@ -965,9 +973,6 @@ import { SvelteDate } from 'svelte/reactivity';
 						</div>
 
 						<!-- Pay button -->
-						{#if payButtonError}
-							<p class="text-sm text-red-600">{payButtonError}</p>
-						{/if}
 						<Button
 							onclick={() => (paymentType === 'deposit' ? handlePayDeposit() : handlePayFull())}
 							class="w-full"
@@ -1034,13 +1039,13 @@ import { SvelteDate } from 'svelte/reactivity';
 							disabled={status !== 'idle'}
 									/>
 								</div>
+								{#if partialValidationError}
+									<p class="mt-1 text-sm text-red-600">{partialValidationError}</p>
+								{/if}
 							</div>
 						{/if}
 
 						<!-- Pay button -->
-						{#if payButtonError}
-							<p class="text-sm text-red-600">{payButtonError}</p>
-						{/if}
 						<Button
 							onclick={() => (paymentType === 'partial' ? handlePayPartial() : handlePayFull())}
 							class="w-full"