Fix ComfyUI external model-path discovery and downloads

[calmasacow]

Summary

This PR fixes setup-wizard model handling when ComfyUI uses extra_model_paths.yaml for external model storage.

Changes

• Parse ComfyUI extra_model_paths.yaml, including base_path and category paths.
• Detect existing models in configured external model directories.
• Keep workflows visible when models are missing, while auto-selecting workflows whose requirements are detected.
• Use the configured external category directory as the destination for missing-model downloads.
• Fall back to the selected ComfyUI installation only when no external category path is configured.

Validation

• Production build passes.
• Existing Qwen GGUF was detected from the external model storage.
• A model download was verified in the YAML-configured external folder rather than the local ComfyUI models folder.