fix: add troubleshooting

[tasshi-me]

Why

Adding a Troubleshooting section improves usability and helps users quickly resolve common errors without needing to search external resources.

What

• Add a Troubleshooting section to the README (ja, en)
  • Covers common issues such as:
    • Connection errors (base URL, proxy, retry)
    • Authentication errors (credentials, secure access domain)
    • Permission errors (access rights, guest space, API token scopes)

How to test

• Check that the new “Troubleshooting” section renders correctly in both README.md and README_en.md
• Confirm internal links and formatting (e.g., headings, code blocks) display properly on GitHub

Checklist

• Updated documentation if it is required.
• Added tests if it is required.
• Passed pnpm lint and pnpm test on the root directory.