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

[katsushi2441]

Summary

• add a first-class ja locale with complete parity against the existing English translation key tree
• restore the desktop and mobile language selector and persist Japanese selection
• localize the landing experience, trader terminal, chart dates, and TradingView locale
• allow strategy configs and the default-strategy API to preserve ja
• request Japanese human-readable reasoning while keeping JSON keys, XML tags, symbols, and action enums unchanged
• add Japanese clamp warnings and Telegram language selection, status, and help
• document how Japanese decision output preserves the machine-readable trading contract

Safety

The system prompt schema remains English and action values such as open_long, open_short, and hold are unchanged. Only user-facing natural-language values are requested in Japanese, so parser and execution behavior stay stable.

Validation

• npm test (131 tests)
• npm run build
• ESLint on all changed frontend files with the repository's existing Prettier baseline excluded
• go test ./kernel ./store ./telegram ./api
• go vet ./kernel ./store ./telegram ./api
• Playwright desktop and mobile checks for language selection, persistence, Japanese landing content, and zero page errors

A full go test ./... run encountered the existing live Coinank websocket test failure once; rerunning go test ./provider/coinank/coinank_api passed.

Closes #1522

[cla-assistant]

All committers have signed the CLA.