fix: modernize react i18next compatibility

[devakone]

Summary

• Updates package compatibility metadata and dev dependencies for modern React, Formik, i18next, and react-i18next versions.
• Fixes public source exports to use the existing kebab-case hook filenames.
• Stabilizes i18n languageChanged cleanup handlers and forces Formik validation for translated touched-field errors.
• Adds Vitest coverage for src/dist exports, Formik error revalidation behavior, listener cleanup, and data-i18n HTML translation.

Validation

• npm test
• npm pack --dry-run
• node -e "const lib = require('./dist'); console.log(Object.keys(lib).sort().join(','))"

Notes

• npm test now runs npm run build && vitest run.
• The package still publishes only README, LICENSE, package metadata, and dist files via the existing files allowlist.