Scale Advanced disclosure chevrons with typography

[Lyptik]

Summary

Small visual refinement for Advanced disclosures.

• make the Advanced chevron smaller than the top-level section chevron
• derive both chevron size and stroke from the current text size using shared em-based disclosure variables
  • top-level sections use the section title size
  • Advanced automatically scales down with the Advanced title size
• shorten the leading divider before Advanced using the same typographic scale
• keep the divider starting at the section-content edge
• no runtime or layout behavior changes

This keeps the relationship rule-based rather than hardcoding separate pixel sizes for each disclosure level.