The active conversation state previously failed WCAG 1.4.11 Non-text Contrast
requirements (~1.2:1 contrast ratio). Added a left border indicator using
border-xheavy which provides 6:1+ contrast in both light and dark modes.
- Add border-l-2 with border-xheavy color for active state
- Conditionally apply rounded-r-lg (active) vs rounded-lg (inactive)
- Use transition-[background-color] to prevent border/radius animation glitch
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