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)

backend/internal/square/square_dev_test.go

@@ -91,8 +91,8 @@ func TestDevClient_CreateCheckout_PendingThenCompleted(t *testing.T) {
 		return getErr == nil && completed.Status == "COMPLETED"
 	}, 5*time.Second, 100*time.Millisecond, "expected checkout to complete")
 
-	assert.Equal(t, int64(8000), completed.Amount, "expected amount 8000 (7500 + 500 tip)")
-	assert.Equal(t, int64(500), completed.TipAmount)
+	assert.Equal(t, int64(7500), completed.Amount, "expected amount 7500 (real Square does not add a tip to the checkout amount)")
+	assert.Equal(t, int64(0), completed.TipAmount)
 
 	// The completed terminal payment's SquarePayID must equal its ID too
 	// (parity with paymentFromSquare), so a terminal-checkout payment recorded