Update available Google models

[Saul-BT]

This pull request updates the references to Google models (changed in the code in #28), the .env file, and the README.
Until now, the documentation and the .env example pointed to outdated models that are no longer available, making it impossible for Claude Code to work if you followed the instructions as written.

With this pull request:

• The code, .env example, and README are now aligned with the currently available Google models.
• New users won’t get stuck with errors when trying to use the models.
• Improves clarity and ensures the project works correctly out-of-the-box.

[winstondu]

@Saul-BT , can you add the gemini 3 preview models too?

[mohanarpit]

@winstondu Opened a PR for Gemini 3 support at #62