Fix: pre-fill payment amount when invoice is selected via radio button

The radio input uses wire:model.live which only sets the property but
doesn't call selectInvoice(). Added updatedSelectedInvoiceId hook so
the amount defaults to the full balance due whenever an invoice is picked.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Livewire/Receptionist/CollectPaymentWizard.php

@@ -231,6 +231,16 @@ private function advanceEnrollmentBillingDate(Enrollment $enrollment): void
         $enrollment->update(['next_billing_date' => $next->toDateString()]);
     }
 
+    public function updatedSelectedInvoiceId(): void
+    {
+        if ($this->selected_invoice_id) {
+            $invoice = Invoice::find($this->selected_invoice_id);
+            if ($invoice) {
+                $this->payment_amount_display = number_format($invoice->due_amount / 100, 2, '.', '');
+            }
+        }
+    }
+
     public function selectInvoice(int $id): void
     {
         $this->selected_invoice_id = $id;