[Don't merge] spring-26-vite downstream

[robsimmons]

Additions:

• A Vite application that lives entirely in the frontend directory.

Notable changes:

• The Express server implements a pattern where it serves the Vite-derived frontend only in production mode. In development mode, anyone accessing the Express server is told to run and use the Vite development server instead. During development mode, the Vite development server proxies /api calls to the Express server.