Add refund action to booking list/detail with full-refund toggle

- New RefundBookingTableAction on the booking list row and detail page,
  refunding a Confirmed booking directly via RefundBookingAction — no need
  to hunt up its Payment on the Refunds resource first.
- Payment::refundableBalance() extracted from RefundBookingAction's private
  balance check so both refund forms can display and cap against it.
- RefundBookingAction::resolveRefundablePayment() made public for the same
  reason (round-trip leg resolution reused by the UI).
- Both refund forms (ProcessRefundAction and the new booking action) gain a
  "Full refund" toggle, on by default, which refunds the payment's whole
  remaining balance without requiring a manually typed amount. Turning it
  off reveals an amount field capped at the refundable balance.

app-modules/payment/tests/Unit/PaymentRefundableBalanceTest.php

@@ -0,0 +1,28 @@
+<?php
+
+use Modules\Payment\Models\Payment;
+use Modules\Payment\Models\Refund;
+
+test('refundable balance is the full amount when nothing has been refunded yet', function () {
+    $payment = Payment::factory()->completed()->create(['amount' => 15000]);
+
+    expect($payment->refundableBalance())->toBe('15000.00');
+});
+
+test('refundable balance subtracts only completed refunds', function () {
+    $payment = Payment::factory()->completed()->create(['amount' => 15000]);
+
+    Refund::factory()->completed()->for($payment)->create(['amount' => 5000]);
+    Refund::factory()->failed()->for($payment)->create(['amount' => 3000]);
+    Refund::factory()->for($payment)->create(['amount' => 2000]); // default state is Pending
+
+    expect($payment->refundableBalance())->toBe('10000.00');
+});
+
+test('refundable balance reaches zero once fully refunded', function () {
+    $payment = Payment::factory()->completed()->create(['amount' => 15000]);
+
+    Refund::factory()->completed()->for($payment)->create(['amount' => 15000]);
+
+    expect($payment->refundableBalance())->toBe('0.00');
+});