Docs: macOS build/packaging caveats for OS-location grid feature (#471)

[Reid-n0rc]

Summary

Document the two macOS build/packaging caveats needed for the proposed OS-location-service grid feature (#471) so they aren't rediscovered the hard way during implementation.

These are runtime/packaging caveats, not compilation blockers — the feature compiles with the existing macOS prerequisites (default clang, Rust, Node; CoreLocation ships in the base macOS SDK, so there's no new tool to install). But it won't work on macOS without them, so they belong in desktop/README.md alongside the existing Windows Smart App Control / Hamlib notes.

Notes to document

1. Info.plist usage-description key — macOS denies location without NSLocationWhenInUseUsageDescription. Set it via tauri.conf.json (bundle.macOS); document why it's required and that the string is user-visible in the permission prompt.

2. Must run as a signed, bundled .app — CoreLocation won't grant location to the bare target/release/ft8af binary, to tauri dev / cargo run, or to an unsigned/ad-hoc build. Testing the feature requires the packaged, signed FT8AF.app. (Same class of caveat as the existing "Windows Smart App Control blocks unsigned binaries" note.)

Scope

• Documentation only (desktop/README.md). No code.
• Relates to Desktop: optionally set operator grid from the computer's OS location service (opt-in) #471 (the feature itself).