Add GitHub Actions workflow for iOS app build

[macterra]

Add a manual GitHub Actions workflow to build an iOS app from the react-wallet Capacitor project, similar to the existing Android debug APK workflow.

Prerequisites:

• Apple Developer account ($99/year)
• Provisioning profile and signing certificate stored as GitHub secrets
• npx cap add ios and npx cap sync ios to initialize the iOS platform

Notes:

• Uses macos-latest runner
• @capacitor-mlkit/barcode-scanning already supports iOS
• Without a developer account, only simulator builds (not installable on real devices) are possible