Add GGUF image model support in --loadall mode

- Detect if image model is GGUF (ends with .gguf or contains 'gguf')
- If GGUF, load using llama.cpp (same as text Vulkan models)
- If diffusers model, load using Stable Diffusion pipeline
- Fixed both locations where image model preloading happens
- Now supports both GGUF and diffusers image generation models