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>

routes/web.php

@@ -2,6 +2,7 @@
 
 use App\Http\Controllers\Auth\SsoLoginController;
 use App\Http\Controllers\WalletBalanceController;
+use App\Http\Controllers\WalletTopupController;
 use App\Http\Controllers\NotificationController;
 use App\Http\Controllers\Public\QrScanController;
 use App\Http\Controllers\Public\TransferPublicController;
@@ -38,6 +39,7 @@ Route::get('/q/{shortCode}/transfer/files/{file}', [TransferPublicController::cl
 
 Route::middleware(['auth', 'platform.session'])->group(function () {
     Route::get('/wallet/balance', [WalletBalanceController::class, 'balance'])->name('wallet.balance');
+    Route::post('/wallet/topup', [WalletTopupController::class, 'store'])->name('user.wallet.topup');
     Route::get('/notifications', [NotificationController::class, 'index'])->name('notifications.index');
     Route::get('/notifications/unread', [NotificationController::class, 'unread'])->name('notifications.unread');
     Route::post('/notifications/{id}/read', [NotificationController::class, 'markAsRead'])->name('notifications.mark-read');