Fix POS checkout: set issue_date and due_date when creating invoices

InvoiceService::create() was not setting issue_date or due_date,
causing a NOT NULL violation on the invoices table when the POS
checkout flow creates an invoice without explicitly passing dates.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Domain/Financial/Services/InvoiceService.php

@@ -18,6 +18,8 @@ public function create(array $data, array $items, User $creator): Invoice
         return DB::transaction(function () use ($data, $items, $creator) {
             $invoice = Invoice::create([
                 ...$data,
+                'issue_date' => $data['issue_date'] ?? now()->toDateString(),
+                'due_date' => $data['due_date'] ?? $data['issue_date'] ?? now()->toDateString(),
                 'status' => InvoiceStatus::Draft,
                 'created_by' => $creator->id,
                 'due_amount' => $data['total_amount'] ?? 0,