Add PLASMA_MOBILE_LOCAL_KAUTH_INSTALL CMake option to redirect KAuth
helper executables, D-Bus service files, and polkit policy files into
CMAKE_INSTALL_PREFIX instead of hardcoded /usr paths.
Guard KF6Auth find_package calls in helper subdirectories with
if(NOT TARGET KF6::AuthCore) to avoid re-running find_package after
the top-level override, which would revert generated service Exec
paths back to /usr/libexec/kf6/kauth.