feat: make avatar size configurable via config.toml

[janjagusch]

Summary

• Adds avatar_size option to the [UI] section of config.toml to allow users to customize the message avatar size in pixels
• Default remains 20px for backwards compatibility
• Users can now set any pixel value (e.g., 32, 40) for larger, more detailed avatars

Usage

[UI]
# Avatar size in pixels (default: 20)
avatar_size = 32

Test plan

• Verify default avatar size (20px) works without config change
• Verify custom avatar size (e.g., 32px) applies correctly
• Verify error avatar icon also respects the configured size
• Verify config change triggers UI update

Closes #2743

🤖 Generated with Claude Code

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Summary by cubic

Make avatar size configurable via [UI].avatar_size in config.toml. Default stays 20px and the size applies to message avatars and the error icon.

• New Features
  • Add UI.avatar_size (pixels) to config.toml; default 20.
  • Frontend uses the value for avatars and the AlertCircle error icon.
  • Update backend UISettings and react-client types.

^{Written for commit 7f0c8d2. Summary will update on new commits.}