Default model indicator icon is confusing and inconsistently positioned

Steps to reproduce

1. Open a Chat in Duo Chat
2. Open a model selection dropdown
3. Observe the icon used to indicate the default model
4. Notice the icon's position relative to other models in the list

Current behavior

• The default model uses an unclear icon that doesn't clearly communicate "default" status
• The icon position is inconsistent, appears on the left side in some contexts and right side in others
• Users may not understand what the icon represents without additional context

Expected behavior

• Replace the ambiguous icon with clear text indicator: add "(default)" after the model name in a badge
• Maintain consistent positioning of the default indicator across all model selection interfaces
• The text-based approach is more universally understood and accessible