style: fix prefer-const and prettier formatting issues

frontend/src/lib/components/payments/PaymentModal.svelte

@@ -15,7 +15,7 @@
 		onComplete: (payment: PaymentResult) => void;
 	}
 
-	let { booking, onClose, onComplete }: Props = $props();
+	const { booking, onClose, onComplete }: Props = $props();
 
 	type PaymentStatus =
 		| 'idle'
@@ -47,17 +47,17 @@
 	let paymentResult = $state<PaymentResult | null>(null);
 	let error = $state<string | null>(null);
 
-	let stamps = $derived(booking.user?.loyalty_stamps ?? 0);
+	const stamps = $derived(booking.user?.loyalty_stamps ?? 0);
 	let useLoyalty = $state(false);
 
-	let loyaltyEligible = $derived(
+	const loyaltyEligible = $derived(
 		stamps >= 10 &&
 			!(booking.discounts ?? []).some((d: BookingDiscount) => d.discount_source === 'loyalty') &&
 			booking.total_amount > 0 &&
 			booking.amount_paid === 0
 	);
 
-	let loyaltyDiscount = $derived(
+	const loyaltyDiscount = $derived(
 		useLoyalty ? Math.round(booking.total_amount * 100 * LOYALTY_DISCOUNT_RATE) : 0
 	);
 
@@ -203,13 +203,15 @@
 		return service.override_price ?? service.price ?? 0;
 	}
 
-	let subtotal = $derived((booking.services ?? []).reduce((sum, s) => sum + getServicePrice(s), 0));
-	let discountSum = $derived(
+	const subtotal = $derived(
+		(booking.services ?? []).reduce((sum, s) => sum + getServicePrice(s), 0)
+	);
+	const discountSum = $derived(
 		(booking.discounts ?? []).reduce((sum, d) => sum + d.discount_amount, 0)
 	);
-	let netTotal = $derived(Math.max(0, subtotal - discountSum));
+	const netTotal = $derived(Math.max(0, subtotal - discountSum));
 
-	let tipPercentages = $derived.by(() => {
+	const tipPercentages = $derived.by(() => {
 		if (netTotal <= 0) return [];
 		return [
 			{ pct: 10, amount: Math.round(netTotal * 0.1 * 100) / 100 },
@@ -218,7 +220,7 @@
 		];
 	});
 
-	let tipMultiplier = $derived(
+	const tipMultiplier = $derived(
 		selectedTipPercent !== null
 			? 1 + selectedTipPercent / 100
 			: customTipAmount && parseFloat(customTipAmount) > 0
@@ -226,8 +228,8 @@
 				: 1
 	);
 
-	let totalWithTip = $derived(tipEnabled ? netTotal * tipMultiplier : netTotal);
-	let tipDisplay = $derived(
+	const totalWithTip = $derived(tipEnabled ? netTotal * tipMultiplier : netTotal);
+	const tipDisplay = $derived(
 		selectedTipPercent !== null
 			? `${selectedTipPercent}%`
 			: customTipAmount && parseFloat(customTipAmount) > 0
@@ -235,7 +237,7 @@
 				: ''
 	);
 
-	let totalDue = $derived(tipEnabled ? totalWithTip : netTotal);
+	const totalDue = $derived(tipEnabled ? totalWithTip : netTotal);
 
 	function formatCurrency(value: number): string {
 		return new Intl.NumberFormat('en-GB', {
@@ -378,8 +380,8 @@
 	});
 
 	let cashAmount = $state<string>('');
-	let cashAmountNum = $derived(cashAmount === '' ? 0 : parseFloat(cashAmount));
-	let changeDue = $derived(cashAmountNum > totalDue ? cashAmountNum - totalDue : 0);
+	const cashAmountNum = $derived(cashAmount === '' ? 0 : parseFloat(cashAmount));
+	const changeDue = $derived(cashAmountNum > totalDue ? cashAmountNum - totalDue : 0);
 	let extraAsTip = $state(false);
 
 	function handleCashInput(e: Event) {
@@ -511,7 +513,7 @@
 		giftCardId = formatted;
 	}
 
-	let giftCardValid = $derived(useAccountBalance || giftCardId.replace(/-/g, '').length === 12);
+	const giftCardValid = $derived(useAccountBalance || giftCardId.replace(/-/g, '').length === 12);
 
 	async function handleGiftCardPayment() {
 		if (!giftCardValid) {