feat(i18n): add Japanese UI and localized decision output

Problem

NOFX already provides Japanese README documentation, but the product UI currently supports only en, zh, and id. Strategy configuration accepts only en/zh, and Japanese users cannot select Japanese output for AI decision explanations.

Proposed change

• Add ja to the web language selector and persisted language context.
• Add a complete Japanese translation dictionary with key/placeholder parity tests.
• Keep the trading prompt contract, JSON keys, action enums, symbols, and deterministic risk controls in English.
• When a strategy uses language ja, request Japanese for user-facing reasoning/output values only.
• Accept ja in strategy defaults and preview endpoints.
• Add Japanese to Telegram's language selection.
• Preserve English fallback for future untranslated keys.

This deliberately avoids translating machine-readable trading contracts, so localization does not change execution semantics.

Testing

• Frontend translation key/placeholder parity tests.
• Frontend typecheck/build and tests.
• Go tests for Japanese strategy normalization and output-language instruction.