ci(ios): bump to Xcode 26 / macOS 15 for iOS 26 SDK requirement

[JamieRuderman]

Summary

• App Store validation now rejects builds compiled with anything older than the iOS 26 SDK (Xcode 26+). Our CI was on Xcode 16 (iOS 18.2 SDK), causing altool --validate-app to fail with error 409.
• Bumps the runner from macos-14 → macos-15 (required for Xcode 26) and the Xcode action from '16' → '26'.
• No app-side changes: IPHONEOS_DEPLOYMENT_TARGET stays at 13.0, certs/provisioning profiles unaffected.

Test plan

• Run the Build / iOS workflow manually and confirm the archive, export, validate, and upload steps all pass
• If pod install or build fails on the new toolchain, may need to bump Capacitor plugins or the Podfile platform :ios minimum

[aws-amplify-us-west-1]

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