Refine video guide with editorial typography

[TREAFREE]

Summary

• replace the generic sans-serif presentation with a system serif/宋体 editorial hierarchy
• add a conversion ledger that explains the product at a glance
• alternate tutorial chapters and add player chrome plus format tags
• retain the notch-inspired navigation and reduced-motion behavior

Verification

• rendered and visually reviewed at 1440x1100, 1440x4200, and 500x900 in Chrome
• five video/source tags and all 12 local references resolve
• all five demos report H.264
• git diff --check passes