Add email translator core engine

[nonggde]

Closes #495

Summary

• add a folder-local deterministic Email Translator core service
• expose local APIs for language detection, supported pairs, validation, loading/error state, single translation, and batch translation
• add synthetic translation fixtures plus Node tests for request contracts, deterministic output, detection, validation, and batch summaries
• document the input/output/loading/error contract in a new core contract note

Scope

• only touches tools/v2/individual/email-translator/
• no app shell, routing, auth, wallet, Stellar, database, inbox, mail rendering, or design-system integration
• no live network calls, secrets, production data, LLM calls, persistence, or side effects

Validation

• node --test tools/v2/individual/email-translator/tests/email-translator-core.test.mjs
• npx --yes prettier --check on the new service, test, fixture, docs, and index files
• import smoke check for tools/v2/individual/email-translator/index.mjs
• git diff --check