Workflows with names containing e.g. a colon character fail silently on save to disk

### Expected Behavior

I expected either an error message or a successful save to disk.

### Actual Behavior

While experimenting with the new UI, I saved some workflows with names containing colons, e.g. flux:name1, sdxl:name2, sd15:name3, etc. The UI allowed the naming using colons, and apparently also allowed saving them to disk. After "saving" them this way, I was able to switch between the workflows using the sidebar menu, which further led me to believe that they were safely saved to disk. But on the next Comfy reload, they were all gone, and the /user/default/workflows folder just contained three empty files "flux", "sdxl", "sd15" (note no .json), one for each colon prefix. I suspect any forbidden filesystem character will trigger this problem but I only tried the colon.

### Steps to Reproduce

1. Save a workflow containing a colon, e.g. "aaa:bbb".
2. The UI allows the save and the sidebar can switch to and from the new workflow as usual.
3. But it has not been saved. The /user/default/workflows folder now has a 0 kB file called "aaa".

### Debug Logs

```powershell
nothing logged in the console
```

### Other

Please prioritize fixing this bug. Silent save errors that cause data loss are among the worst class of bugs.