Two-step add-funds modal instead of wallet redirect on insufficient balance

When a user hits an action they can't afford (e.g. creating a QR code with
0/low balance), open an in-app two-step modal instead of bouncing them to the
central wallet page. Step 1 explains pay-as-you-go billing + shows the action
cost and current balance; step 2 takes an amount and starts a Paystack checkout
via the central identity wallet-topup API, returning the user where they were.

- service-topup-modal: rebuilt as two-step (billing explainer -> amount/pay)
- IdentityClient::walletTopup + WalletTopupController + POST /wallet/topup
- balanceGate.openTopup / qrCustomizer.redirectTopup now prefer the modal,
  falling back to the wallet page only when no modal is wired
- qr-codes index + create render the modal and open it on insufficient balance

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

resources/views/qr-codes/create.blade.php

@@ -15,6 +15,7 @@
             balance: @js((float) $wallet->spendableBalance()),
             price: @js((float) $pricePerQr),
             topupUrl: @js($topupUrl),
+            topupModalId: 'qr',
             type: @js(old('type', $prefill['type'] ?? $defaultType ?? 'url')),
          })">
 
@@ -254,5 +255,17 @@
             </div>
         </div>
 
+        {{-- Two-step add-funds modal (insufficient balance opens this instead of redirecting to the wallet) --}}
+        <x-user.service-topup-modal
+            id="qr"
+            title="Add funds"
+            :topup-action="route('user.wallet.topup')"
+            :price-per-action="(float) $pricePerQr"
+            action-noun="QR code"
+            :service-balance="(float) $wallet->spendableBalance()"
+            service-balance-label="Wallet balance"
+            :return-url="route('user.qr-codes.create')"
+            :open-on-load="session('open_topup_modal') === 'qr'" />
+
     </div>
 </x-user-layout>