Task: Switch to `pnpm` for JS dep management

## Describe the task
We are currently using `npm` for dependency management for FE development. Although this works, `pnpm` is now the preferred package manager for working with Vercel (which we do), and is generally more robust and faster than `npm`. Switch over our dependency management to use `pnpm` instead of `npm`, updating all documentation as necessary and flagging any issues that arise.

No files, tests, or entry points are named. Locate the frontend dependency manifests, npm-based setup commands, and related documentation; verify the existing development workflow before updating references, and consider the task done when pnpm is used consistently and any migration issues are recorded.

Written by the indexing model from the issue text.