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/today/CurrentAppointment.svelte

@@ -6,6 +6,7 @@
 	import { Button } from '$lib/components/ui/button';
 	import { Badge } from '$lib/components/ui/badge';
 	import { Skeleton } from '$lib/components/ui/skeleton';
+	import PaymentModal from '$lib/components/payments/PaymentModal.svelte';
 
 	interface Props {
 		openBookingModal: (bookingId: string) => void;
@@ -44,6 +45,7 @@
 	let loading = $state(true);
 	let timeRemaining = $state(0); // minutes remaining in current appointment
 	let isInProgress = $state(false);
+	let showPaymentModal = $state(false);
 
 	// Calculate time remaining and free time
 	let interval: ReturnType<typeof setInterval> | null = null;
@@ -163,7 +165,12 @@
 	}
 
 	function handleTakePayment() {
-		toast.info('Take payment - Coming soon');
+		showPaymentModal = true;
+	}
+
+	function handlePaymentComplete(payment: unknown) {
+		toast.success('Payment completed');
+		showPaymentModal = false;
 	}
 
 	function handleCancel() {
@@ -425,3 +432,7 @@
 		</Card.Content>
 	{/if}
 </Card.Root>
+
+{#if showPaymentModal}
+	<PaymentModal booking={activeAppointment} onClose={() => showPaymentModal = false} onComplete={handlePaymentComplete} />
+{/if}