Add Japanese (ja) localization

[monta-gh]

Summary

This PR adds Japanese (ja) localization to Toolbox.

Changes

• Added assets/translations/ja.i18n.json (full Japanese translation, 698 strings)
• Added ja to CFBundleLocalizations in ios/Runner/Info.plist

Translation Workflow

1. Repository & source analysis
2. Pre-translation analysis
3. AI translation
4. AI review
5. Native human review

[Koizeay]

Hello,
Thank you for your work on adding Japanese localization.

After review, the changes have been approved,

Please note that the app is constantly being updated, so I'll try to add Japanese translations for new tools and features

PS : I will add your GitHub username to the app's credits about the Japanese localization.

[monta-gh]

Thank you for merging and for adding my name to the credits! Looking forward to seeing the Japanese localization in the app.