[Cleanup] Rename multi-user feature to multi-profile

The current user login feature concept conflicts with the API Nodes account login feature concept. This issue proposes we rename the existing multi-user user login to multi-profile to avoid confusion.

Actionables:
- Deprecate `--multi-user` argument in the backend server. Replace it with `--multi-profile` and prompt existing users which passes `--multi-user` arg to move.
- Rename the `User` concept used in current frontend to `Profile`, e.g. https://github.com/Comfy-Org/ComfyUI_frontend/blob/main/src/views/UserSelectView.vue

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-3488-Cleanup-Rename-multi-user-feature-to-multi-profile-1d86d73d36508113b78cec1e2aa17398) by [Unito](https://www.unito.io)