[FEATURE]: Add Gemini sign-in instead of requiring an API key

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

It would be really helpful if OpenCode could support signing in with a Google/Gemini account instead of requiring users to manually add a Gemini API key.

A lot of people already have a Google AI Pro subscription, and it would be great if they could simply sign in with their Google account and use the Gemini models included with their subscription.

Having to create an API key separately feels like an extra step, especially for users who are already paying for Google AI Pro. A “Sign in with Google” / Gemini login option would make the setup much easier and more accessible.

This would be especially useful for people who already have an AI Pro plan and would love to use their existing Gemini access directly in OpenCode, without having to deal with API keys and separate API billing.

I think this could make the Gemini experience much smoother for a lot of users. 🙏
Thanks for considering it!