feat: Square payment integration, booking flow redesign, and timezone/weekday fixes

- Add Square payment integration (mock + handlers + UI): terminal/online payments,
  refunds, tips, saved cards, webhooks. Build-tagged dev/prod clients.
- Redesign booking flow: Step 4 conditional (deposit only), Step 5 confirmation
  screen with booking ID, auto-submit on transition.
- Redesign schedule modal: 2x3 button grid with Pay Deposit/Pay Early logic.
- Add deposit warning banner at Step 1 for users with outstanding deposits.
- Fix weekday conversion bug: Go 0=Sunday vs DB 0=Monday mismatch in 6 locations.
- Fix timezone bug: UTC vs London time in closing hours validation.
- Fix frontend error parsing: plain text backend errors now displayed correctly.
- Fix crypto.randomUUID fallback for environments without Web Crypto.
- Add 7 new regression tests: closing hours, advance check, active booking limit,
  weekday conversion, UTC/London, deposit snapshot, exceptional hours.
- Fix 3 flaky tests: dynamic dates instead of fixed, no-show timing.

frontend/src/lib/components/account/UserBookingModal.svelte

@@ -8,6 +8,7 @@
 	import * as Textarea from '$lib/components/ui/textarea';
 	import * as Label from '$lib/components/ui/label';
 	import DatePicker from '$lib/components/booking/DatePicker.svelte';
+	import PaymentModal from '$lib/components/payments/PaymentModal.svelte';
 	import type { Booking, WorkingHoursDay, AvailableHoursDay } from '$lib/types/booking';
 	import { extractBookedSlots, getLunchProtectionForSlots } from '$lib/lunchProtection';
 
@@ -94,6 +95,25 @@
 			.reduce((sum, p) => sum + p.amount, 0) || 0
 	);
 
+	let depositOutstanding = $derived(
+		selectedBooking?.deposit_required && !selectedBooking?.deposit_paid
+	);
+
+	let canPayEarly = $derived(
+		selectedBooking &&
+		!depositOutstanding &&
+		totalPaid < selectedBooking.total_amount &&
+		['confirmed', 'pending'].includes(selectedBooking.status)
+	);
+
+	let showPaymentModal = $state(false);
+
+	function handlePaymentComplete() {
+		toast.success('Payment completed');
+		showPaymentModal = false;
+		fetchBookingDetails();
+	}
+
 	let isRescheduleValid = $derived(
 		rescheduleDate && rescheduleTime && rescheduleTime.length >= 4
 	);
@@ -847,12 +867,37 @@
 						Add to Calendar
 					</Button>
 				{/if}
-				<Button size="sm" class="flex-1" onclick={() => (open = false)}>Close</Button>
+				{#if depositOutstanding}
+					<Button
+						size="sm"
+						class="flex-1 bg-amber-600 hover:bg-amber-700 text-white"
+						onclick={() => (showPaymentModal = true)}
+					>
+						Pay Deposit
+					</Button>
+				{:else if canPayEarly}
+					<Button
+						size="sm"
+						class="flex-1 bg-emerald-600 hover:bg-emerald-700 text-white"
+						onclick={() => (showPaymentModal = true)}
+					>
+						Pay Early
+					</Button>
+				{/if}
 			</div>
+			<Button size="sm" class="w-full" variant="ghost" onclick={() => (open = false)}>Close</Button>
 		</div>
 	</Modal.Content>
 </Modal.Root>
 
+{#if showPaymentModal && selectedBooking}
+	<PaymentModal
+		booking={selectedBooking}
+		onClose={() => (showPaymentModal = false)}
+		onComplete={handlePaymentComplete}
+	/>
+{/if}
+
 <Modal.Root open={showCancelConfirm} onOpenChange={(v) => (showCancelConfirm = v)}>
 	<Modal.Content class="max-w-sm">
 		<Modal.Header>