mobile/ is a separate pnpm project, so the root pnpm install leaves
mobile/node_modules empty and pnpm build failed in build:mobile-web with
hundreds of unexplained esbuild "Could not resolve" errors.
build-mobile-web-app-bundle now checks for mobile/node_modules first and
exits with the one command that fixes it. The probe path is shared with
the bundle tests' dependency check. CONTRIBUTING documents the install in
Local Setup and points "Before Opening a PR" at it.
Fixes#24324
Co-authored-by: Neil <neil@stably.ai>