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/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">