Model Downloads Stall, Do Not Resume, and Leave Partial Files That Block Redownload

## Describe the bug

When downloading large model files, AI Playground sometimes stalls mid‑download (speed drops to 0.0B/s indefinitely). When this happens:

- The backend repeatedly checks Hugging Face metadata but never retries the download.
- The partially downloaded file is stored inside a randomly named “_tmp” folder.
- The software does not resume the download.
- The software does not overwrite the partial file.
- The software does not delete the partial file.
- Attempting to download again results in a “missing model” popup followed by “Download model file failed.”

This leaves the user stuck until they manually locate and delete the hidden temporary folder.

This issue has existed in earlier versions and is still present in AI Playground 3.0.2 beta.

## To Reproduce

1. Open Image Gen or any workflow that requires downloading a new model.
2. Trigger a model download (e.g., FLUX‑related models or text encoders).
3. Wait until the download stalls mid‑file (speed becomes 0.0B/s).
4. Attempt to restart the download.
5. Observe:

- repeated metadata checks
- “missing model” popup
- “Download model file failed”
- inability to resume or overwrite the partial file

The partial file remains in a hidden folder such as:
C:\Users\\AppData\Local\Programs\AI Playground\resources\models\ComfyUI\clip\_tmp\split_files\text_encoders\

## Expected behavior

The software should:

- Retry the download when it stalls.
- Resume from the last downloaded byte.
- Overwrite or clean up partial files.
- Delete temporary folders after a failed download.
- Allow the user to restart the download without manual file deletion.

Instead, the app becomes stuck until the user manually finds and deletes the hidden “_tmp” folder.

## Environment (please complete the following information):

OS: Windows 11
GPU: Intel Arc A750
CPU: Intel i3‑10100
Version: AI Playground 3.0.2 beta

## Additional context

**Where the bug occurs**

The stalled file was found inside:
...ComfyUI\clip\_tmp\split_files\text_encoders\

This confirms that:

- AI Playground downloads into a temporary folder ending in “_tmp.”

- If the download stalls, the temporary folder is never cleaned up.

- The backend refuses to resume or overwrite the partial file.

- The UI cannot proceed because the model is “missing,” but the partial file blocks re‑download.

**Why this is a problem:**

- Users cannot recover from a failed download without manually searching for hidden folders.

- Large models (3–4 GB) frequently stall due to CDN issues, making this bug disruptive.

- The lack of resume/cleanup logic causes repeated failures and confusion.

**Suggested fixes:**

1. Add retry logic when speed remains 0.0B/s for a certain duration.
2. Add resume support using HTTP range requests.
3. Clean up partial downloads automatically.
4. Overwrite partial files when restarting a download.
5. Improve error messaging to indicate when a download has stalled or cannot resume.