Sign in with ChatGPT (Codex) / Claude Code subscription instead of an API key

Right now, using OpenAI or Anthropic models in Vowen means pasting in a paid API key. Many of us already pay for ChatGPT Plus/Pro or Claude Pro/Max, and tools like the Codex CLI and Claude Code let you sign in with that subscription via OAuth.

It would be great if Vowen supported the same login flow (or used an existing local Codex / Claude Code CLI login) as an alternative to an API key. That way we could use the subscription we already pay for instead of being billed again per token.

Suggested options:

• "Sign in with ChatGPT" (Codex-style OAuth) for OpenAI models

• "Sign in with Claude" (Claude Code-style OAuth) for Anthropic models

• Or detect and use an existing codex / claude CLI login on the machine

API keys would stay available for anyone who prefers them.