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>

app/Services/Identity/IdentityClient.php

@@ -43,6 +43,24 @@ class IdentityClient
         return (array) $response->json('data', []);
     }
 
+    /**
+     * Start a central-wallet Paystack top-up for an account and return the
+     * Paystack checkout URL. Backs the in-app "Add funds" modal so users top up
+     * without leaving the app for the central wallet page.
+     */
+    public function walletTopup(string $publicId, float $amount, string $returnUrl): string
+    {
+        $response = $this->request()->post($this->url('/identity/wallet-topup-account'), [
+            'user' => $publicId,
+            'amount' => $amount,
+            'return_url' => $returnUrl,
+        ]);
+
+        $response->throw();
+
+        return (string) $response->json('data.checkout_url', '');
+    }
+
     private function request()
     {
         return Http::withToken((string) config('identity.api_key'))