feat: Integrate with OpenRouter to facilitate GPT4V access

[louisgv]

Hi @steveruizok,

Stumbled upon your tweet this morning, and what you described is basically what I and @alexanderatallah have been working on over the past few months!

This PR replaces the current BYOK input flow with an OAuth virtual key flow through OpenRouter. The end-user can set spending limits and revoke the key on either end. We can remove the header that tracks the app usage as well. Although it's optional, I thought it'd be interesting to see make-real on the leaderboard.

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