Allow custom companion avatar/visual

Feature Request

Current behavior:
The companion character (the small buddy next to the input box) allows customization of name and personality in .claude.json, but the visual representation (cactus icon) is hardcoded and cannot be changed.

Desired behavior:
Allow users to customize the companion's visual appearance — either by:

1. Choosing from a set of built-in avatars (robot, animal, object, etc.)
2. Providing a custom emoji or icon via config (e.g., "avatar": "🤖")
3. Providing a custom image path (e.g., "avatar": "/path/to/icon.png")

Use case:
I've renamed my companion to J.A.R.V.I.S. with a dry, butler-like robot personality — but the cactus visual doesn't match at all. Being able to set a robot icon (or any custom visual) would make the companion feel much more personal and coherent.

Suggested config addition:

"companion": {
  "name": "J.A.R.V.I.S.",
  "personality": "...",
  "avatar": "🤖",
  "hatchedAt": 1775242336568
}

This would be a small change with a big impact on personalization.