we want to begin front-end development right away

I think that beginning of the development is pretty easy task. Maintaining and modifying existing codebase it’s hard. If project is starting and codebase is small you can rewrite whole project from scratch in few days. But when project grows it is wiser to refactor than rewriting. But if you want to refactor your project has to be maintainable.

We know most of the front-end interactions we want, but more will be discovered and some might be changed.

You can make quick prototypes before you begin serious coding. Start new branch in your repository and don’t try to write some beautiful code. Just hack and tweak features. Mock some features if you need it. I think that prototype phase / sandbox phase is often necessary to gain technical knowledge and gather business requirements, to decide which features are useful and which are not (off course you may need to consult these decisions with your stakeholders, your client, etc. But having working prototype makes this easier than operate on *.psd based designs and unclear guidelines).

And after prototype phase you can delete your sandbox branch and start some serious coding for production.