feat(frontend): add email check and gift card support to booking flow

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

frontend/src/lib/components/admin/UserModal.svelte

@@ -92,6 +92,7 @@
 	let hasEligiblePatchTests = $state(false);
 	let customerRelationship = $state<CustomerRelationship | null>(null);
 	let loadingRelationship = $state(false);
+	let giftCardBalance = $state<number | null>(null);
 
 	async function fetchUserDetails() {
 		if (!userId) return;
@@ -217,11 +218,25 @@
 		}
 	}
 
+	async function fetchGiftCardBalance() {
+		if (!userId) return;
+		try {
+			const res = await fetch(`/api/admin/users/${userId}/giftcard-balance`, {
+				headers: { Authorization: `Bearer ${authStore.currentToken}` }
+			});
+			if (res.ok) {
+				const data = await res.json();
+				giftCardBalance = data.balance;
+			}
+		} catch { /* ignore */ }
+	}
+
 	$effect(() => {
 		if (open && userId) {
 			fetchUserDetails();
 			fetchUserBookings();
 			fetchCustomerRelationship();
+			fetchGiftCardBalance();
 		}
 	});
 
@@ -519,6 +534,15 @@
 							</div>
 						</div>
 
+						{#if giftCardBalance !== null && giftCardBalance > 0}
+							<div class="mt-3 border-t pt-3">
+								<div class="text-xs text-gray-500">Gift Card Balance</div>
+								<div class="text-2xl font-bold text-green-600">
+									£{giftCardBalance.toFixed(2)}
+								</div>
+							</div>
+						{/if}
+
 						{#if customerRelationship.topServices && customerRelationship.topServices.length > 0}
 							<div class="mt-4">
 								<div class="mb-2 text-xs font-semibold text-gray-600">Most Booked Services</div>