i18n: add Japanese (日本語) translation

[AugustoL]

Summary

Add Japanese language support to OpenScan.

Prerequisites

• feat: implement i18n infrastructure with react-i18next #130 (i18n infrastructure) must be completed first

Scope

Translate all user-facing strings to Japanese (日本語), including:

• Navigation and common UI elements
• Address page content
• Block page content
• Transaction page content
• DevTools section
• Error messages
• Settings page

Files to create

src/locales/ja/
  common.json
  address.json
  block.json
  transaction.json
  devtools.json
  errors.json
  network.json

Guidelines

• Use appropriate formality level (polite/です・ます form recommended)
• Keep technical blockchain terms in English where commonly used in Japanese crypto community
• Use katakana for common English loanwords
• Maintain consistent terminology throughout
• Test all translations in context

Acceptance Criteria

• All translation files created
• All strings translated
• UI tested with Japanese selected
• No truncation or layout issues
• Reviewed by native speaker (if possible)

Resources

• Japanese blockchain terminology
• Base English files in src/locales/en/