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>
