fix: make nix dependency Unix-only for Windows compilation (#68)

The nix crate is Unix-only and was listed as an unconditional dependency
in the cli Cargo.toml, preventing compilation on Windows. The actual
usage in main.rs is already gated behind #[cfg(unix)], so this just
moves the dep to [target.'cfg(unix)'.dependencies] to match.

Co-authored-by: nullislander <nullislander@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 3 people

src-rust/crates/cli/Cargo.toml

@@ -38,12 +38,14 @@ url = { workspace = true }
 crossterm = { workspace = true }
 parking_lot = { workspace = true }
 dirs = { workspace = true }
-nix = { workspace = true }
 base64 = "0.22"
 sha2 = "0.10"
 open = "5"
 urlencoding = "2"
 xxhash-rust = { version = "0.8", features = ["xxh64"] }
 
+[target.'cfg(unix)'.dependencies]
+nix = { workspace = true }
+
 [build-dependencies]
 chrono = { workspace = true }