feat: Jan allows bulk import and subfolder recognition for model management

[eckartal]

Problem
The current model import feature in Jan only supports importing one model at a time and does not recognize models stored in subfolders. This limitation affects users who maintain organized collections of models in nested directory structures.

Success Criteria
Develop and implement a feature that allows users to specify a root directory for model imports. Jan should then scan the specified directory and all subdirectories, automatically importing all detected model files.

Additional context
Detailed feedback: https://www.reddit.com/r/LocalLLaMA/comments/1cclmjk/comment/l1byjva/?context=3