refresh council defaults and branding

[laceyp99]

Summary

• Refresh the initial council lineup and fallback catalog with Gemini 3.5 Flash Lite, GPT-5.6 Luna, and Claude Sonnet 5.
• Add the application icon assets and serve the primary icon as the Gradio favicon.
• Cover the revised defaults and favicon launch configuration with tests.

Testing

• .\venv\Scripts\python.exe -m ruff format --check . - passed
• .\venv\Scripts\python.exe -m ruff check arena tests - passed
• .\venv\Scripts\python.exe -m pytest - passed