New BYOK models: GPT-5.5 / 5.4 and Gemini 3.5 Flash / 3.1 Pro

Kai now supports the latest AI models when using your own API key (BYOK):

• OpenAI: GPT-5.5, GPT-5.4, GPT-5.4 Mini
• Google: Gemini 3.5 Flash, Gemini 3.1 Pro

The model picker was also reorganized: newest models appear first within each provider, and older models (Gemini 2.5 Flash/Pro, GPT-4.1, GPT-5 Mini, GPT-5.1, GPT-5.2) moved into a collapsed Legacy Models section. Existing selections keep working.