Part of #430.

Remove the React PWA from the monorepo. Prerequisite: the discontinuation banner from the pre-pivot sub-issue has shipped.

Tasks

• Remove app/ directory entirely
• Remove app/prepare-resources.js and any root scripts that only existed for the app
• Update turbo.json to drop app-specific tasks and env passthroughs
• Drop app from the root package.json workspaces list
• Update root README.md: remove "web app" narrative, describe new model (convention + linter + optional stack)