Function move_window_by 

pub fn move_window_by(
    state: &Arc<AppState>,
    args: &Value,
) -> Result<Value, String>

Move the window by a delta (dx, dy) from its current position.

Prefer set_window_position for drag flows. This function reads the current rect via GetWindowRect then SetWindowPos with the new origin — under rapid concurrent calls (mousemove drag), in-flight IPCs all read the same stale rect and only one delta gets applied. set_window_position is self-contained (no read-modify-write) and idempotent under concurrency.