The frontend fix alone wasn't enough because:
1. serveIndexHtml() was injecting the raw avatar filename into HTML
2. agent.identity.get RPC was returning raw filename, overwriting the
HTML-injected value
Now both paths resolve local file avatars (*.png, *.jpg, etc.) to the
/avatar/{agentId} endpoint URL.
Adds the ability to customize the assistant's name and avatar in the Web UI.
Configuration options:
- config.ui.assistant.name: Custom name (replaces 'Assistant')
- config.ui.assistant.avatar: Emoji or letter for avatar (replaces 'A')
Also reads from workspace IDENTITY.md as fallback:
- Name: field sets the assistant name
- Emoji: field sets the avatar
Priority: config > IDENTITY.md > defaults
Closes#1383
