docs: add Japanese README translation

[sscodeai]

Summary

Add README_ja.md and wire up the language switcher in README.md / README_zh.md so the existing trilingual setup actually includes Japanese.

What's included

• New file: README_ja.md — full Japanese translation of the README, structurally aligned with the English version and including the mobile section that exists in README_zh.md
• Updated language switcher in README.md and README_zh.md to expose the 日本語 link (the bar previously only linked between EN ↔ ZH)

Translation notes

• Section structure and ordering follow the English README
• The mobile preview section that's only present in README_zh.md was also included so the Japanese version isn't a strict subset of either source
• Localized professional terms to match README_zh.md's conventions:
  • super administrator / regular administrator → スーパー管理者 / 一般管理者
  • multi-agent extensibility → マルチ Agent 拡張アーキテクチャ
• Kept English as-is for product/protocol terms (Profile, Provider, Backend, bearer token, Bot token, etc.) since they appear that way in the product UI and platform docs
• Token is capitalized when used as a standalone noun (matching README_zh.md); kept lowercase inside fixed phrases like Bot token / Access token / bearer token
• All environment variables, CLI commands, and code blocks are kept verbatim

Test plan

• Markdown renders correctly on GitHub
• Language switcher links resolve in all three READMEs (EN ↔ ZH ↔ JA)
• Section structure and anchor links match the English version
• No broken image / video references