Add File Management Features and Improve Presets Handling

[nabilaba]

This pull request introduces several quality-of-life features, especially useful for users running on server-based environments like Google Colab and Kaggle, where restarting the runtime isn't practical:

✅ Download button for retrieving files (models, LoRAs, embeddings, VAEs) directly from the interface
✅ Delete button to remove files including models, LoRAs, embeddings, VAEs, and presets
✅ Backup and restore functionality for presets
✅ File refresh now updates the file list in real time without requiring a restart
🧹 Removed base model (no error)

These changes help users add, remove, and manage files dynamically, reducing the need to restart sessions—which is a common limitation in one-time execution environments like Colab and Kaggle.