Doesn't seem to be a facility to resume incomplete model downloads

To clarify, I'm opening a workflow and using the Missing Models error report to begin the download.

Comfy's models are typically around 32GB, therefore requiring download times of more than an hour. So it's not surprising that downloads will frequently end in an incomplete state.

The download manager current has a facility to intentionally pause downloading of a file, but seems to have no facility to resume a download that was **not** intentionally paused.

I've tried probably about 8 times to download some of the larger models. It almost completes, reaching random percentages such as 93 and 98 complete. But at that point something hangs. It is not possible to add another download, or to resume the original download. Instead, it's necessary to close and restart comfyUI -- by which point the partial download has been lost.

It seems it ought to be possible to at the very least save the partial download (up to the point where it has been reliably retrieved) as a temporary file, and to offer to resume the download from there. Or, maybe, integrate a trustworthy open source resumable downloader into comfy's model manager so that it can perform the download with comfy only needing to organise the correct place to save it.

pa -- firefox managed the download OK, but lacks the convenience that Comfy's model downloader was intended to provide.

Start from the Missing Models error report and the download manager's existing pause handling, then trace how incomplete downloads are discarded when the manager hangs or ComfyUI restarts. Done means a partially retrieved model can be retained and resumed without losing progress or blocking subsequent downloads.

Written by the indexing model from the issue text.