(feat): Improve CMake build configuration for consistent cross-platform GPU support

[Montana]

Hey team,

The current CMake configuration in vpic-kokkos does not consistently detect or configure GPU backends (CUDA, HIP, SYCL) across platforms. In some environments this leads to:

• GPU support being silently disabled
• confusing or opaque build failures
• inconsistent configurations depending on compiler and Kokkos backend selection

More robust GPU backend detection and reporting would improve portability on HPC systems and reduce onboarding friction for new users.

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Behavior

Running a standard configure step:

cmake -B build -S .

may result in one or more of the following depending on the host environment:

• available GPU platforms are not detected
• the build falls back to CPU-only mode without a warning
• target GPU architectures are misconfigured (e.g., sm_70 vs sm_80 on NVIDIA)

In these cases it is not always obvious to the user whether GPU acceleration is actually enabled.

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Suggested Improvements

A more explicit CMake configuration flow could:

• clearly report which Kokkos backend(s) were detected and selected
• fail early if a requested backend is unavailable or improperly configured
• document or print any expected environment variables / toolchain requirements

For example, improvements might include:

• summary messages during configuration indicating backend status
• explicit options such as
  -DKokkos_ENABLE_CUDA=ON, -DKokkos_ENABLE_HIP=ON, -DKokkos_ENABLE_SYCL=ON
• sanity checks when a backend is requested but not supported on the system

Michael.