fix style

frontend/src/pages/ChallengeDetail.jsx

@@ -196,9 +196,31 @@ export default function ChallengeDetail() {
           </div>
         )}
 
-        <div style={{ display: 'grid', gap: '2rem', gridTemplateColumns: 'minmax(0, 1fr) 300px' }}>
+        {/* Leaderboard - Desktop: Sidebar, Mobile: Top */}
+        <div className="card" style={{ marginBottom: '2rem' }} id="leaderboard-mobile">
+          <h3 style={{ marginBottom: '1rem' }}>Leaderboard</h3>
+          {leaderboard.length === 0 ? (
+            <p style={{ color: 'var(--text-muted)', fontSize: '0.875rem' }}>No points yet</p>
+          ) : (
+            <div style={{ display: 'flex', flexDirection: 'column', gap: '0.75rem' }}>
+              {leaderboard.map((entry, index) => (
+                <div key={entry.id} style={{ display: 'flex', justifyContent: 'space-between', alignItems: 'center' }}>
+                  <div>
+                    <span style={{ marginRight: '0.5rem', color: 'var(--text-muted)' }}>{index + 1}.</span>
+                    <strong>{entry.username}</strong>
+                  </div>
+                  <span style={{ color: 'var(--primary)', fontWeight: 600 }}>
+                    {entry.validated_points} pts
+                  </span>
+                </div>
+              ))}
+            </div>
+          )}
+        </div>
+
+        <div className="challenge-detail-layout">
           {/* Main Content */}
-          <div>
+          <div className="challenge-main">
             {/* New Prediction */}
             <div className="card" style={{ marginBottom: '2rem' }}>
               <h3 style={{ marginBottom: '1rem' }}>Make a Prediction</h3>
@@ -272,8 +294,8 @@ export default function ChallengeDetail() {
             )}
           </div>
 
-          {/* Sidebar - Leaderboard */}
-          <div>
+          {/* Sidebar - Leaderboard (Desktop only) */}
+          <div className="challenge-sidebar">
             <div className="card">
               <h3 style={{ marginBottom: '1rem' }}>Leaderboard</h3>
               {leaderboard.length === 0 ? (