Transfer: two-step add-funds modal on insufficient balance

Creating a transfer that the wallet can't cover used to flash a bare error.
Detect the insufficient-balance case and open an in-app two-step modal (billing
explainer -> amount -> Paystack) on the create page instead, with an 'Add funds'
button to top up on demand.

- BillingClient::topup -> POST /api/billing/topup (Transfer already holds its key)
- WalletTopupController + route POST /wallet/topup (user.wallet.topup)
- store() flashes open_topup_modal on TransferWalletErrors::isInsufficientBalance
- service-topup-modal made two-step (uses <x-modal>, plain POST -> checkout)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/Http/Controllers/Transfer/TransferController.php

@@ -4,10 +4,12 @@ namespace App\Http\Controllers\Transfer;
 
 use App\Http\Controllers\Controller;
 use App\Models\Transfer;
+use App\Services\Billing\BillingClient;
 use App\Services\Qr\QrImageGeneratorService;
 use App\Services\Qr\QrPdfExporter;
 use App\Services\Transfer\TransferService;
 use App\Services\Upload\ChunkedUploadService;
+use App\Support\TransferWalletErrors;
 use Illuminate\Http\RedirectResponse;
 use Illuminate\Http\Request;
 use Illuminate\Support\Str;
@@ -59,12 +61,21 @@ class TransferController extends Controller
         return view('transfer.transfers.index', compact('transfers', 'search', 'status', 'sort'));
     }
 
-    public function create(): View
+    public function create(BillingClient $billing): View
     {
+        $account = ladill_account();
+        $balanceCedis = 0.0;
+        try {
+            $balanceCedis = $billing->balanceMinor((string) $account->public_id) / 100;
+        } catch (\Throwable) {
+            // Balance is informational on this page; ignore lookup failures.
+        }
+
         return view('transfer.transfers.create', [
             'maxFiles' => (int) config('transfer.max_files_per_transfer', 20),
             'pricePerGb' => (float) config('transfer.price_per_gb_month', 0.30),
             'gracePeriodDays' => (int) config('transfer.grace_period_days', 15),
+            'balanceCedis' => $balanceCedis,
         ]);
     }
 
@@ -108,7 +119,12 @@ class TransferController extends Controller
         try {
             $transfer = $this->transfers->create($account, $data, $files);
         } catch (RuntimeException $e) {
-            return back()->withInput()->with('error', $e->getMessage());
+            $redirect = back()->withInput()->with('error', $e->getMessage());
+            if (TransferWalletErrors::isInsufficientBalance($e)) {
+                $redirect->with('open_topup_modal', 'transfer');
+            }
+
+            return $redirect;
         } finally {
             foreach ($uploadIds as $uploadId) {
                 $this->chunkedUploads->cleanup($uploadId);