[Feature]: Support displaying custom agent names in the chat dialog and allow custom agent avatars via user-provided image URL.

## Summary

Support displaying custom agent names in the chat dialog and allow custom agent avatars via user-provided image URL.

## Component(s) Affected

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

## Problem / Motivation

At present, the chat interface only shows the default name and avatar, making it difficult to distinguish different agents. Users need personalized display to improve recognition and experience in multi-agent scenarios.

## Proposed Solution

1. Add one configuration fields when creating/editing an agent:
- Custom avatar URL (supports image bed address input)
2. Render the custom name and avatar in the chat dialog by priority; use default values if not configured.