Remove thick top border from challenge cards, use uniform thin border

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: etzm

assets/css/style.css

@@ -313,16 +313,10 @@ ul {
   transform: translateY(-2px);
 }
 
-.challenge-challenge {
-  border-top: 3px solid #9e9e9e;
-}
-
-.challenge-solution {
-  border-top: 3px solid #9e9e9e;
-}
-
+.challenge-challenge,
+.challenge-solution,
 .challenge-impact {
-  border-top: 3px solid #9e9e9e;
+  border-top: none;
 }
 
 .challenge-icon {