- Add 'all' as a valid backend option
- Change default from 'nvidia' to 'all'
- Add comprehensive 'all' backend section that installs:
  - Base requirements
  - PyTorch with CUDA (nvidia backend)
  - llama-cpp-python with CUDA and Vulkan support
  - stable-diffusion-cpp-python with OpenCL
  - Additional requirements
- Detect available hardware (CUDA, Vulkan, OpenCL) and enable accordingly
- Show summary of available backends after installation