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/src/Models/Payment.php

@@ -10,6 +10,7 @@ use Modules\Booking\Models\Booking;
 use Modules\Payment\Database\Factories\PaymentFactory;
 use Modules\Payment\Enums\PaymentMethod;
 use Modules\Payment\Enums\PaymentStatus;
+use Modules\Payment\Enums\RefundStatus;
 use Spatie\Activitylog\Models\Concerns\LogsActivity;
 use Spatie\Activitylog\Support\LogOptions;
 
@@ -74,4 +75,17 @@ class Payment extends Model
     {
         return $this->hasMany(Refund::class);
     }
+
+    /**
+     * What's left to refund on this Payment — its total minus whatever has
+     * already been completed-refunded (partial refunds supported, domain.md
+     * §6). Shared by RefundBookingAction's own guard and the Filament refund
+     * forms, which surface it to staff before they submit.
+     */
+    public function refundableBalance(): string
+    {
+        $alreadyRefunded = (string) $this->refunds()->where('status', RefundStatus::Completed->value)->sum('amount');
+
+        return bcsub((string) $this->amount, $alreadyRefunded, 2);
+    }
 }