[codex] Add Japanese localization support

[cormoran]

Summary

• add a lightweight language provider, translation dictionary, and header language toggle
• localize the app shell, connection flow, Home, Keymap, Trackball, BLE, Battery, Settings, Custom Subsystems, Health Check, and shared selector components
• persist the selected language in localStorage and update document language

Validation

• npm run lint
• npm run build
• npm test -- --runInBand
• browser check on http://127.0.0.1:5173/ with Japanese toggle and demo-mode Home/Keymap/Trackball

[github-actions]

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_{Deployed from commit cf4263f}