encapsulate the mockdata sanity-checks done in pr_check.sh into a dedicated npm-script, and move them elsewhere.

ideas

• hook it into the verify npm-script flow, so it runs in the proper phase (i.e. at the FE-build-container runtime)
  proved to be problematic - see this thread
• put it in a git-hook, e.g. before-push (while we're at it, document that and other git-hooks in the readme)

considerations

• as pointed out here, these checks should probably not run on CI; they belong to dev' workflow