i18n: tooling, drafts & CI guardrails

[Davidnet]

Part of the frontend internationalization effort (see Epic).

Tasks

• Add eslint-plugin-i18next (opt-in per directory) to flag new hardcoded strings
• CI script checking locale key parity across en/es/fr/ja
• Fill es / fr / ja JSON with drafted (machine) translations for native refinement
• Japanese (CJK) font/layout pass — verify rendering and that longer strings don't break layout (tray menu, dashboard cards)
• Document the i18n workflow in docs/02-development-guide.md