ci: Use latest macOS runner + Xcode to match dev SDK

CI was running on macos-14 with Xcode 16.1. The developer is on
macOS 26 (Tahoe) with Xcode 26.x. Binaries built against the macOS 14
SDK and run on macOS 26 trigger SwiftUI behavior-compat shims —
including MenuBarExtra panel sizing — which produced the visible
empty bands users reported on 1.5.3 / 1.5.4 / 1.5.5 even though the
SwiftUI VStack inside reported the correct rendered size in the log.
Use macos-latest + latest-stable Xcode so the published SDK matches
or is newer than typical user environments.