Detect checkpoints and loras in subfolders

Add a function to detect checkpoints and loras in subfolders, and add folder structure for model management.

I have added comfyui and stable diffusion path to the model management setting, but currently comflowy can only detect models put in the root folder of models.

Or is there any method to fix this in the app?

Look for model management settings and file detection logic in the codebase, likely in a directory like `src/models/` or `src/utils/`. Start by understanding how the app currently scans the root folder for checkpoints and LoRAs. Then, modify the detection function to recursively traverse subfolders. Test by adding models in nested directories and verifying they appear in the UI.

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