docs: update OpenViking Helper downloads

[yufeng201]

Description

Updates the OpenViking Helper README sections for the 0.0.19 beta release. The docs now mention Windows x64 support, refresh the macOS download links, and add the missing Japanese README section.

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• Updated OpenViking Helper platform wording from macOS-only to macOS and Windows x64.
• Replaced macOS download links with the 0.0.19 DMG URLs.
• Added the Windows x64 0.0.19 EXE download URL.
• Added the missing Japanese README Helper section.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Validation performed:

• rg -n "0\\.0\\.18|macOS only|仅支持 macOS|macOS-only|supports macOS only" README.md README_CN.md README_JA.md
• git diff --check

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

N/A

Additional Notes

This PR is documentation-only; no runtime tests were run.