Add Nix dev shell with Xcode support

[tellmeY18]

Add a flake.nix that sets up a reproducible development shell for BitChat on macOS.

[Chessing234]

opened #1582 — intentionally tiny: nix develop for cli helpers + docs that xcode is still required. a full swift/nix app shell would rot against apple sdks; happy to iterate if you want specific packages added.