docs: add authenticated star history chart

[cyfyifanchen]

Summary

• Replace the legacy anonymous Star History SVG with the official sealed-token embed in both English and Chinese READMEs.
• Support light and dark color schemes for the Star History chart.
• Keep the raw GitHub PAT out of the repository. The committed sealed token is the public embed value generated by Star History.

Type

• Fix
• Feature
• Docs
• CI / tooling
• Refactor
• Other

Verification

• Relevant tests pass locally

• Relevant lint / type checks pass locally

• User-facing docs or screenshots are updated when needed

• git diff --check origin/main...HEAD passed.

• make check-large-files passed.

• The interactive Star History chart rendered with the configured token. The embed endpoint also accepted and decrypted the sealed token, but a direct cold request intermittently returned HTTP 500 when its server-side GitHub fetch exceeded 10 seconds.

Risk

• Security impact considered
• Backward compatibility considered
• Rollback path is clear for risky changes

The chart remains dependent on the remote Star History service rendering reliably on GitHub.

Related Issues

Tracks #202