feat(add-on): add ElysiaJS + Eden Treaty

Which project does this relate to?

Create TanStack App

Describe the feature

There are already a lot of great add-ons available for Create TanStack App, and I’d love to see first-class support for ElysiaJS and Eden Treaty added as well. 🩵

ElysiaJS is a fast, TypeScript-first web framework that pairs especially well with the TanStack ecosystem. Eden Treaty provides end-to-end type safety and autocomplete when consuming an Elysia server, making the combination a great fit for full-stack TanStack applications.

An add-on could provide a ready-to-use ElysiaJS backend alongside the TanStack Start frontend, with Eden Treaty configured for type-safe API communication out of the box.

Additional context

Eden Treaty is the recommended client for Elysia's type-safe API experience and provides typed API access directly from the Elysia server definition.

Start by reviewing the existing Create TanStack App add-ons, then read the linked ElysiaJS and Eden Treaty documentation. Define how an ElysiaJS backend and Eden Treaty client should be scaffolded alongside the TanStack Start frontend. Done means the add-on provides that ready-to-use, type-safe setup.