fix: tip double-count, fully-paid auto-completion, discount-refund hardening

Tip double-count (root cause of £33.75 vs £28.75 display):
- Remove mock's fixed +500p auto-tip when AllowTipping is true (square_dev.go) —
  real Square only enables a terminal prompt, it never adds a tip to the amount
- Set AllowTipping=false in CreateTerminalPayment: the frontend already embeds
  the tip in the amount, so the terminal must not prompt for a second tip
- M4 tip split now derives the tip as charged amount minus remaining booking
  value ('after 100% is tips'), not from Square's TipAmount field
- Success screens divide paymentResult.amount by 100 (pence -> pounds) in both
  PaymentModal and UserPaymentModal

Fully-paid bookings auto-complete:
- Extract ApplyBookingCompletionSideEffects into payments package (shared by
  admin progress endpoint and payment paths; avoids circular import)
- Add bookingIsFullyPaid + completeFullyPaidBooking: when completed non-tip
  payments reach 100% of the booking total, an active booking transitions to
  'completed' so it leaves the admin Current Appointment view
- Wired into CreateBookingPayment (inside tx) and GetCheckoutStatus (terminal,
  after commit); completion side-effects (loyalty, campaigns, deposits_required)
  fire identically to the manual progress endpoint
- Add /admin/bookings/{id}/refund route (AdminRefundBooking)

Discount-refund hardening:
- RefundPayment explicitly rejects discount/on_the_house payments (was relying
  on the incidental NULL-square_payment_id guard)
- Hide the Refund button for discount/on_the_house payments in EditBookingModal
- Cancel-refund estimate in BookingModal also excludes on_the_house
- Cancellation refund loop + GetBookingPaymentInfo + GetBookingRefundableAmountCents
  exclude payment_type='tip' from refundable totals

Tip flow (start-time guard) fixes tests:
- Tip tests updated to use past-dated bookings (tips now require booking started)

Tests:
- m4_tip_refund_redesign_test.go (tip split, refund exclusion, admin refund cap)
- m5_fully_paid_completion_test.go (online + terminal full-payment completion,
  partial stays active, tip excluded, cancelled stays cancelled)
- Full suite passes with -race (25 packages)

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

@@ -37,7 +37,12 @@
 	// Derived values for cancel confirmation
 	const totalPaid = $derived(
 		(selectedBooking?.payments ?? [])
-			.filter((p) => p.payment_method !== 'discount' && p.status === 'completed')
+			.filter(
+				(p) =>
+					p.payment_method !== 'discount' &&
+					p.payment_method !== 'on_the_house' &&
+					p.status === 'completed'
+			)
 			.reduce((sum, p) => sum + p.amount, 0)
 	);
 	const totalRefunds = $derived(

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

@@ -606,7 +606,7 @@
 											})}
 										</div>
 									</div>
-									{#if payment.status === 'completed'}
+									{#if payment.status === 'completed' && payment.payment_method !== 'discount' && payment.payment_method !== 'on_the_house'}
 										<Button
 											size="sm"
 											variant="outline"

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

@@ -339,8 +339,11 @@ import { submitPaymentWithRetry } from '$lib/square/square';
 						last4: data.card_last4,
 						amount: data.amount
 					};
-					toast.success('Payment successful');
-					onComplete(paymentResult);
+					// Deliberately do NOT call onComplete() here: it would make
+					// the parent close this modal instantly, so the green-tick
+					// success state would never be seen. The modal stays open
+					// showing the tick until the operator clicks Done, which
+					// fires handleSuccessDone() → onComplete + onClose.
 				} else if (data.status === 'FAILED') {
 					stopPolling();
 					status = 'error';
@@ -368,6 +371,17 @@ import { submitPaymentWithRetry } from '$lib/square/square';
 		onClose();
 	}
 
+	// Called from the success state's Done button: notify the parent (so it can
+	// refresh the booking/payment data) and then close the modal. Kept separate
+	// from handleClose so a success state never closes without the callback.
+	function handleSuccessDone() {
+		stopPolling();
+		if (paymentResult) {
+			onComplete(paymentResult);
+		}
+		onClose();
+	}
+
 	function resetToSelect() {
 		stopPolling();
 		status = 'idle';
@@ -1379,7 +1393,7 @@ import { submitPaymentWithRetry } from '$lib/square/square';
 						<div class="flex justify-between">
 							<span class="text-sm text-gray-600">Amount</span>
 							<span class="font-semibold text-gray-900">
-								{formatCurrency(paymentResult.amount)}
+								{formatCurrency(paymentResult.amount / 100)}
 							</span>
 						</div>
 						{#if paymentResult.card_brand}
@@ -1397,7 +1411,7 @@ import { submitPaymentWithRetry } from '$lib/square/square';
 					</div>
 				</div>
 
-				<Button onclick={handleClose} class="w-full">Done</Button>
+				<Button onclick={handleSuccessDone} class="w-full">Done</Button>
 			</div>
 		{/if}
 	</Dialog.Content>

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

@@ -964,7 +964,7 @@
 						<div class="flex justify-between">
 							<span class="text-sm text-gray-600">Amount</span>
 							<span class="font-semibold text-gray-900">
-								{formatCurrency(paymentResult.amount)}
+								{formatCurrency(paymentResult.amount / 100)}
 							</span>
 						</div>
 						<div class="flex justify-between">