Add macOS app wrapper packaging

[abhinavgautam01]

Summary

• Add a minimal unsigned Stringcast.app wrapper around the Rust CLI binary
• Add macOS bundle metadata via Info.plist
• Add packaging/macos/build_app.sh for local app bundle generation
• Include Stringcast.app in macOS release artifacts
• Document macOS app wrapper build/test steps

Verification

• plutil -lint packaging/macos/Info.plist
• sh -n packaging/macos/build_app.sh
• sh -n packaging/macos/Stringcast
• cargo build --release
• packaging/macos/build_app.sh
• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test
• cargo build
• python3 -m py_compile scripts/wayland_listener.py