style: fix prefer-const and prettier formatting issues

frontend/src/lib/components/booking/BookingFlow.svelte

@@ -72,7 +72,7 @@
 	let selectedServices = $state<Service[]>([]);
 	let selectedDate = $state<CalendarDate | undefined>(undefined);
 	let selectedTime = $state<string | null>(null);
-	let customerInfo = $state<CustomerInfo>({
+	const customerInfo = $state<CustomerInfo>({
 		firstName: '',
 		lastName: '',
 		email: '',
@@ -98,13 +98,13 @@
 	let newCardNumber = $state('');
 	let newCardExpiry = $state('');
 	let newCardCVC = $state('');
-	let saveCardForFuture = $state(false);
+	const saveCardForFuture = $state(false);
 
 	// Payment flow state
 	let depositPaid = $state(false);
 	let showPaymentForm = $state(false);
 
-	let depositCardFormValid = $derived(
+	const depositCardFormValid = $derived(
 		selectedPaymentMethod !== null ||
 			(showNewCardForm &&
 				newCardNumber.replace(/\s/g, '').length >= 13 &&
@@ -113,17 +113,17 @@
 	);
 
 	// VAT registration status from public business info (via shared store)
-	let vatRegistered = $derived(getBusinessInfo()?.is_vat_registered ?? false);
+	const vatRegistered = $derived(getBusinessInfo()?.is_vat_registered ?? false);
 
 	// Email existence check (guest flow only)
 	let emailChecking = $state(false);
 	let emailSuggestion = $state<string | null>(null);
 	let emailError = $state('');
-	let emailFormatValid = $derived(
+	const emailFormatValid = $derived(
 		!customerInfo.email ||
 			/^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/.test(customerInfo.email)
 	);
-	let allGuestFieldsValid = $derived(
+	const allGuestFieldsValid = $derived(
 		customerInfo.firstName &&
 			customerInfo.lastName &&
 			customerInfo.email &&
@@ -571,12 +571,12 @@
 	let loadingWorkingHours = $state<boolean>(false);
 	let loadingAvailableHours = $state<boolean>(false);
 
-	let workingHoursCache: Record<
+	const workingHoursCache: Record<
 		string,
 		Record<string, { isOpen: boolean; startTime: string; endTime: string }>
 	> = {};
 
-	let availableHoursCache: Record<
+	const availableHoursCache: Record<
 		string,
 		Record<string, { isOpen: boolean; slots: Array<{ startTime: string; endTime: string }> }>
 	> = {};
@@ -602,7 +602,7 @@
 	});
 
 	// Track which months are currently being fetched (prevents duplicate requests)
-	let loadingMonths: Record<string, boolean> = {};
+	const loadingMonths: Record<string, boolean> = {};
 
 	// Preload current + next month on first render; subsequent months fetched individually
 	let initialLoadDone = $state(false);
@@ -1304,13 +1304,13 @@
 		selectedDate ? getDayWithOrdinal(selectedDate) : undefined
 	);
 
-	let depositRequired = $derived(calculateDepositRequired());
-	let totalSteps = $derived(authStore.isAuthenticated ? 4 : 5);
+	const depositRequired = $derived(calculateDepositRequired());
+	const totalSteps = $derived(authStore.isAuthenticated ? 4 : 5);
 	// StepIndicator uses displayNumber = startAt + index. currentStep aligns with displayNumber,
 	// not the array index. For auth: startAt=1, totalSteps=4 → last displayNumber=4.
 	// For guest: startAt=0, totalSteps=5 → last displayNumber=4. Always evaluates to 4.
-	let finalStep = $derived(totalSteps - 1 + (authStore.isAuthenticated ? 1 : 0));
-	let stepLabels = $derived(
+	const finalStep = $derived(totalSteps - 1 + (authStore.isAuthenticated ? 1 : 0));
+	const stepLabels = $derived(
 		authStore.isAuthenticated
 			? ['Service', 'Date & Time', 'Details', 'Payment']
 			: userDepositsRequired > 0