The repository had no root README at all, and frontend/README.md was
still the stock Vite template. Add a bilingual (EN/RU) overview covering
the mod, site, and backend plus how they connect (%link, %config
save/load), replace the frontend README with the actual stack, layout,
and scripts, and fix the mod README's broken DEVELOPMENT.md link to
point at DEV_GUIDE.md.
