Fix: Improve sd.cpp model loading fallback logic

- Enhanced model resolution for sd.cpp fallback path
- Added multiple fallback strategies:
  1. Try HuggingFace GGUF resolution (existing)
  2. Fallback to direct file path check
  3. Fallback to cached model lookup
  4. Last resort: attempt download as URL
- Better error logging and handling
- Ensures model loading attempts all possible resolution paths before failing