Unify active section highlight and refine Advanced divider

[Lyptik]

Summary

Follow-up polish based on the latest mobile rendering.

Section selection

• make every open top-level section use the same explicit selected/highlight background
• remove reliance on Safari's transient tap/focus painting
• keep a keyboard-visible focus state
• preserve the subtle hover state on desktop

Advanced divider

• indent Advanced slightly to the right
• change the divider composition to:
  • short line
  • chevron + Advanced
  • long line
  • optional compact value summary
• keep the whole row vertically aligned on Safari/mobile

Audit cleanup

While auditing the current sidebar code:

• removed obsolete pre-accordion .panel styling
• removed unused .text-button styling
• removed unused .sticky-heading styling
• removed an obsolete Advanced hint rule
• found and fixed a real geometry conflict: the Debug sticky-header rule was still overriding shared summary padding, which could make a collapsed Debug row differ in height from the other collapsed sections
• checked the responsive breakpoint: CSS and JS use the same phone/touch-device condition
• checked the current Pages workflow and latest main deployment; both are healthy
• no unused CSS classes remain in the current HTML/JS surface after this cleanup

CSS-only UI change. No tracking, connection, rendering, persistence, or data-processing behavior is changed.