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>

src/main.rs

@@ -238,6 +238,11 @@ fn update_background(
     cols: u16,
     rows: u16,
 ) {
+    // Kitty graphics-protokollet stöds inte av Windows Terminal/conhost —
+    // att skriva APC-sekvenser dit ger bara skräptecken.
+    if cfg!(windows) {
+        return;
+    }
     let wanted_path = app.config.background_image.as_deref();
 
     // Kolla om vi behöver göra något