Add three onboarding improvements for the Astrai provider:
- Signup URL: users now see "Get your API key at: https://as-trai.com"
during onboarding instead of a blank prompt
- Curated model list: auto (best execution), GPT-4o, Claude Sonnet 4.5,
DeepSeek V3, Llama 3.3 70B
- Live model fetch: Astrai's OpenAI-compatible /v1/models endpoint is
now queried when an API key is present, matching other providers
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>