feat(i18n): add Japanese (ja-JP) locale

[YizukiAme]

Summary

Add complete Japanese (ja-JP) localization for OpenMAIC.

Changes

• [NEW] lib/i18n/locales/ja-JP.json — Full Japanese translation of all 785 keys
• [MODIFY] lib/i18n/locales.ts — Register ja-JP in supportedLocales

Translation Notes

• Translated from en-US.json with reference to zh-CN.json for context
• Natural Japanese phrasing used throughout (not machine-translation style)
• Appropriate politeness levels (丁寧語) for an educational platform
• All {{interpolation}} variables preserved exactly

Verification

• All 785 keys present — 0 missing, 0 extra (verified via script)
• JSON syntax valid (node -e "require('./lib/i18n/locales/ja-JP.json')")
• TypeScript compiles (npx tsc --noEmit passes)
• Locally tested — language switcher shows 日本語, full UI renders in Japanese
• No English fallback text observed in primary UI

Screenshots

Language Switcher	Japanese Homepage

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have added/updated documentation as needed
• My changes do not introduce new warnings