feat(onboarding): simplify first provider setup

[esafwan]

Summary

• add a prominent “Get started with AI” entry point to AI Providers
• guide a first administrator through OpenRouter Free or Google AI Studio / Gemini
• save the provider key, reuse the seeded provider/model where available, and create the starter model when needed
• explain rate, availability, and privacy caveats instead of presenting free options as production-ready

Why

Huf seeds a provider/model catalog, but a new user still has to discover the key → model → agent sequence and know valid model identifiers. The existing CRUD pages do not expose a safe starter path.

Notes

• OpenRouter uses openrouter/free, a dynamic free router. It is explicitly labeled for experimentation and demos.
• Gemini uses gemini-2.5-flash and explains the free-tier data-use caveat.
• This saves credentials in Huf's existing encrypted Password field but does not test credentials. A provider-aware server-side connection test is intentionally follow-up work.

Validation

• yarn typecheck
• yarn test — 67 tests passing
• yarn build

yarn lint remains red on the existing repository-wide baseline (409 existing violations); no lint errors were introduced by this change.