chore: Update Japanese translations for new features and errors

[dai]

Added and updated multiple Japanese locale strings across AI, app, errors, external, settings, and shortcuts modules. Changes include new labels, descriptions, error messages, and UI text for features such as scheduling, token usage, inbox management, toolbar customization, and quick search.<!-- DO NOT IGNORE THE TEMPLATE!

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チームのみなさん、こんにちは。久々に時間が作れたので日本語翻訳を一通りレビューしました。 加えて日本語版README（README-ja.md）も作ったのでお使いください。

Hello, everyone on the team. I finally found some time to thoroughly review the Japanese translations. Additionally, I've created a Japanese README (README-ja.md), so please feel free to use it.

Description

PR Type

• [ ] Feature
• [ ] Bugfix
• [ ] Hotfix
• [ ] Other (please describe):

Screenshots (if UI change)

Demo Video (if new feature)

Linked Issues

Additional context

Changelog

• [ ] I have updated the changelog/next.md with my changes.

[follow-reviewer-bot[bot]]

Thank you for your contribution. We will review it promptly.

[safedep[bot]]

SafeDep Report Summary

No dependency changes detected. Nothing to scan.

_{This report is generated by SafeDep Github App}

[vercel[bot]]

@dai is attempting to deploy a commit to the RSS3 Team on Vercel.

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[DIYgod]

Thank you for your contribution, but could you omit the README-ja.md for now? We’re planning major changes to the README, and maintaining multiple language versions would complicate future updates.