Replace Yarn usage with pnpm in CI workflows to standardize package
management and leverage pnpm's store/cache behavior.
- Use pnpm/action-setup to install pnpm (v) instead of enabling corepack
and preparing Yarn.
- Retrieve pnpm store path and update cache actions to cache the pnpm
store and use pnpm-lock.yaml for cache keys and restores.
- Replace yarn commands with pnpm equivalents across workflows:
install, i18n:sync/translate, format, build:* and tsx invocation.
- Avoid committing lockfile changes by resetting pnpm-lock.yaml instead
of yarn.lock when checking for changes.
- Update install flags: use pnpm install --frozen-lockfile / --install
semantics where appropriate.
These changes unify dependency tooling, improve caching correctness,
and ensure CI uses pnpm-specific lockfile and cache paths.
