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]

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

[safedep]

SafeDep Report Summary

No dependency changes detected. Nothing to scan.

_{This report is generated by SafeDep Github App}

[vercel]

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

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[Copilot]

Pull Request Overview

This PR adds and updates Japanese translations across multiple modules to support new features and improve localization. The changes include translations for AI scheduling, token usage monitoring, inbox management, toolbar customization, quick search functionality, and MCP service integration. Additionally, a comprehensive Japanese README has been added to improve accessibility for Japanese-speaking users.

Key changes:

• Added Japanese translations for new AI features (scheduling, token usage, MCP integration)
• Updated error messages and UI labels across multiple locale files
• Created a complete Japanese README (README-ja.md) documenting the project

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
locales/shortcuts/ja.json	Added quick search command translations
locales/settings/ja.json	Added toolbar customization, subscription tabs, and token usage translations
locales/external/ja.json	Added "last used" login label
locales/errors/ja.json	Added new error codes for invalid input, missing config, MCP errors, and AI attachment limits
locales/app/ja.json	Added translations for AI summary, inbox management, timeline hiding, and user menu items
locales/ai/ja.json	Added extensive translations for scheduling features, MCP service management, security disclosures, and token usage monitoring
README-ja.md	Created comprehensive Japanese README with project description, installation instructions, and feature highlights

[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.