Investigative work on gemini integration

[devoncarew]

Some investigative work on gemini integration:

• add a 'gemini' button on the main toolbar
• have a 'generate with gemini' model dialog open
• allow the user to describe the app to generate; send that description and a prompt to gemini; replace the dartpad sample with the generated code

This is not enabled by default; it may be useful to discuss the best way to allow trusted testers to opt-into it.

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.