[MAINT] Keep CONTRIBUTING.md, README, and `.nvmrc` aligned with tooling

[krwg]

Summary

Whenever package.json scripts, Node engines, or dev commands change, refresh CONTRIBUTING.md, README (EN + RU Quick start), and .nvmrc so first-time contributors never hit version skew.

Acceptance criteria

• Single source of truth for Node: package.json engines + .nvmrc + README tables match.
• New npm script added → CONTRIBUTING documents it or links to package.json discovery.
• PR template checkbox references this policy.

Notes

Low-effort hygiene; batch with any tooling PR.