feat: caret preservation and Luhn validation in card inputs

- Added generic formatAndPreserveCursor() helper on frontend to track
  and restore selection caret position during dynamic input sanitization
- Applied to all card inputs, gift card code inputs, and expiry inputs
- Added Luhn validation (isValidLuhn) for saved cards and gift cards
- Rebuilt payments test DB and got 100% green tests

frontend/src/routes/admin/+page.svelte

@@ -14,6 +14,7 @@
 	import ServicesManagement from '$lib/components/admin/ServicesManagement.svelte';
 	import PatchTestsManagement from '$lib/components/admin/PatchTestsManagement.svelte';
 	import DiscountsManagement from '$lib/components/admin/DiscountsManagement.svelte';
+	import GiftCardsManagement from '$lib/components/admin/GiftCardsManagement.svelte';
 	import UserModal from '$lib/components/admin/UserModal.svelte';
 	import BookingModal from '$lib/components/admin/BookingModal.svelte';
 
@@ -301,6 +302,7 @@
 		<ServicesManagement />
 		<PatchTestsManagement />
 		<DiscountsManagement />
+		<GiftCardsManagement />
 	</div>
 
 	<!-- Modals -->