Document HTA risks and harden local page handling

[papanda925]

Summary

• rewrite the README as a Japanese-first legacy-project guide
• clearly recommend the PowerShell localhost-server successor for new use
• document HTA/mshta security implications, backup, migration, limitations, and troubleshooting
• resolve the data folder from the HTA file location instead of the current working directory
• validate page names before building file paths
• use the textarea value when saving edited content
• scope the current page variable correctly
• prevent rename overwrite and deletion of the top page
• add Node.js syntax validation for inline HTA JavaScript and repository convention checks

Validation

• CP932 source round-trip passed
• extracted inline JavaScript passes node --check
• local page-name, save behavior, and documentation checks passed
• Windows HTA runtime testing remains required, so this PR is Draft

Project status

This change does not modernize the MSHTML/ActiveX runtime or bundled JavaScript libraries. It documents the legacy status and applies bounded safety fixes while preserving the original architecture.