Cross-platform IPC via interprocess + Windows support fixes

- Replace Unix-socket-only server/client with interprocess local sockets:
  unchanged unix domain socket on Linux (SOCKET_API compatible),
  named pipes on Windows
- Run command lines with arguments via sh -c / cmd /C in the PTY
- Wire RunProgram args into the spawned command
- Make libc a unix-only dependency
- Skip Kitty background image on Windows (protocol unsupported)
- Drop dead pixel-size query code in kitty_gfx
- Reject left/right panel positions at config parse instead of silently
  ignoring them

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Cargo.toml

@@ -18,4 +18,7 @@ portable-pty = "0.9"
 vt100 = "0.16"
 anyhow = "1"
 image = { version = "0.25", default-features = false, features = ["jpeg", "png", "gif", "bmp", "webp"] }
+interprocess = "2.4"
+
+[target.'cfg(unix)'.dependencies]
 libc = "0.2"