Add macOS app target

[setoelkahfi]

Summary

• add a native macOS app target and scheme via XcodeGen
• keep the shared SwiftUI source tree for feature parity with the iOS app
• add lightweight cross-platform view helpers so navigation, onboarding presentation, and toolbar actions work on macOS without breaking iOS
• add a macOS CI build job and refresh the onboarding snapshot after copy changes

MVP scope

• native macOS app target: Klepon Mac
• shared content browsing, search, saved items, settings, and private Onde guide flow
• device-neutral private-guide copy instead of iPhone-only wording
• macOS-specific app plist with minimal app metadata

Implementation notes

• Klepon Mac links SystemConfiguration.framework to satisfy the Onde binary dependency on macOS
• the mac target avoids the watch dependency and uses the same shared Klepon/ source tree
• iOS snapshot tests still pass on the pinned iPhone 17 / iOS 26.2 simulator destination

Verification

• xcodegen generate
• xcodebuild -project Klepon.xcodeproj -scheme 'Klepon Mac' -resolvePackageDependencies
• xcodebuild build -project Klepon.xcodeproj -scheme 'Klepon Mac' -destination 'platform=macOS' CODE_SIGNING_ALLOWED=NO
• xcodebuild test -project Klepon.xcodeproj -scheme Klepon -destination 'platform=iOS Simulator,OS=26.2,name=iPhone 17' -only-testing:KleponSnapshotTests CODE_SIGNING_ALLOWED=NO