add an examples app that has a gallery of pre-designed surfaces and allows editing/previewing A2UI JSON

[andrewkolos]

Fixes #788. Fixes #714.

Adds a new composer app that provides the following:

1. The ability to experiment with generating a GenUI surface using the core catalog. This is done with a simple text prompt. (works only if GEMINI_API_KEY is available in the local environment).
2. A curated gallery/catalog of simple surfaces that use the core catalog. Can be useful when evaluating local changes to the styling/implementation of core catalog widgets.
3. The ability to see/edit the backing JSON of surfaces.

⚠️ For reviewers

The count of changed files is near 100, but most of these are project scaffolding. You can focus on the dart source of the composer app.

There are some open questions I left unanswered in the interest of limiting the scope of this PR--the Files Changed count is already bloated from adding a new app. The big one worth mentioning is: should this new app be merged with catalog_gallery? The gallery of individual components could simply be another tab in the UI. The other thing that catalog_gallery does is act as a suite of integration tests for CI. We could probably do the same thing with this new app: de-dupe redundant samples and port integration tests from catalog_gallery. I'm happy to do this in a follow-up PR.

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I have added sample code updates to the changelog.
• I updated/added relevant documentation (doc comments with ///).

If you need help, consider asking for advice on the #hackers-devrel channel on Discord.

[andrewkolos]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces an excellent composer example app, which will be a valuable tool for developing and testing A2UI surfaces. The app is well-structured, featuring AI-powered UI generation, a gallery of examples, and a live JSON editor. The overall implementation is solid. My feedback includes a few minor suggestions for improvement related to error handling, logging, and code clarity to enhance robustness and maintainability. The included fix for data model notifications in the genui package is also a great improvement.

_{Note: Security Review did not run due to the size of the PR.}