feat: center About Us on the Humanly Lab mission

[ShenzheZhu]

Closes #1025

Summary

• reframe About Us around Humanly Lab's long-term human-authenticity mission
• define authenticity as visible human contribution and AI participation, not AI absence
• replace product/commercial copy with three lab principles: evidence over inference, transparency over prohibition, and human agency at the center
• retain institution and team information while tightening the page spacing
• update English, Simplified Chinese, Japanese, Spanish, and French copy
• rename the English navigation, footer, and page metadata label to “About Us”
• clarify the Open Source pricing description as the current open-source version of Humanly

Verification

• pnpm --filter @humanly/frontend-user type-check
• pnpm --filter @humanly/frontend-user build
• git diff --check
• local visual review at http://localhost:3002/about

Scope

The planned “Beyond Post-hoc Detection” manifesto remains a separate follow-up.