[FR] Searchable existing-user selector on multi-user login

### Is there an existing issue for this?

- [x] I have searched the existing issues and checked the recent builds/commits

### What problem are you experiencing?

In multi-user server mode, the login screen requires selecting an existing user from a dropdown list. When there are many configured users, its hard to find the intended username, as it relies on manually opening the dropdown and scrolling through a long list.

### When does this problem occur?

- When ComfyUI runs in `--multi-user` mode and the frontend shows the user selection/login view.
- Especially noticeable when the number of existing users is large (e.g., dozens to hundreds), and the desired user is not near the top of the list.
- Common workflows: Shared servers with many accounts (team/studio/lab environments)

### How often do you encounter this problem?

Daily

### How much does this problem affect your workflow?

Causes moderate inconvenience

### Current workarounds

- Manually scroll the dropdown list and visually scan for the username.

### Ideas for solutions (Optional)

A user picker that supports typing to filter/search usernames.

### Additional context

_No response_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-7555-FR-Searchable-existing-user-selector-on-multi-user-login-2cb6d73d365081c5a9eff3fd9a7982bf) by [Unito](https://www.unito.io)