[Feature]: support OpenWebUI as a channel

## Summary

I would like to have the OpenWebUI as one of the available channels.

## Component(s) Affected

- [ ] Core / Backend (app, agents, config, providers, utils, local_models)
- [ ] Console (frontend web UI)
- [x] Channels (DingTalk, Feishu, QQ, Discord, iMessage, etc.)
- [ ] Skills
- [ ] CLI
- [ ] Documentation (website)
- [ ] Tests
- [ ] CI/CD
- [ ] Scripts / Deploy

## Problem / Motivation

I want a dedicated AI app to chat with my CoPaw, both on PC and Phone, not a limited messenger. And currently the only viable option to do so seems to be OpenWebUI
OpenWebUI is a bit complex to setup, especially to connect with a phone, but I did it via Tailscale and it works. The only weak point for now is no support for OpenWebUI as a channel from CoPaw.. Would like it to support model/agent selection, proper tool calls info (could do so partly, half-baked still), etc.

## Proposed Solution

I tried to vibe-code my custom channel for it and it kinda works, but very unstable yet and lacks some features. Can share here eventually if it's gonna be implemented. Also probably OpenClaw already has it and implementation can be looked at there.

[Screenshots, examples, links to docs or similar features elsewhere.]

- [ ] I am willing to open a PR for this feature (after discussion).