Save file (workflows and subgraph library) doesn't work

### Prerequisites

- [x] I am running the latest version of ComfyUI
- [x] I have searched existing issues to make sure this isn't a duplicate
- [x] I have tested with all custom nodes disabled ([see how](https://docs.comfy.org/troubleshooting/custom-node-issues#step-1%3A-test-with-all-custom-nodes-disabled))

### What happened?

When trying to save a workflow I get an error message:

Somehow ComfyUI tries to save a file with name "workflows/default.json". In api.ts:946:

`/userdata/${encodeURIComponent(file)}?overwrite=${options.overwrite}&full_info=${options.full_info}`

the file parameter is URL encoded, leading to this error. Maybe the path should be excluded?

Best regards
Alexander

### How is this affecting you?

Feature doesn't work as expected

### ComfyUI Frontend Version

v1.27.2

### Browser

Chrome/Chromium

### Console Errors

```javascript

```

### Logs

```shell

```

### Additional Context
ComfyUI version: 0.3.57
OS posix
Python Version 3.12.11 (main, Jun 4 2025, 08:56:18) [GCC 11.4.0]
Embedded Python false
Pytorch Version 2.4.1+cu124
Arguments main.py --disable-auto-launch --disable-cuda-malloc --listen --multi-user --front-end-version Comfy-Org/ComfyUI_frontend@latest --port 18188
RAM Total 251.2 GB
RAM Free 241.92 GB
_No response_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-5479-Save-file-workflows-and-subgraph-library-doesn-t-work-26a6d73d36508114a5bfcfbc8b8a5c4a) by [Unito](https://www.unito.io)

Read api.ts around line 946 and trace how the file parameter is constructed for workflow and subgraph saves. Verify the request path handling for names such as workflows/default.json, then confirm that saving these files works without the reported error.

Written by the indexing model from the issue text.