Fix last lap purple highlighting to use backend fastest lap flag

Purple now only shows when the backend confirms the driver holds the
fastest lap AND their last lap equals their personal best. Prevents
disagreement between frontend calculation and backend data source.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: adn8naiagent

frontend/src/components/Leaderboard.tsx

@@ -320,17 +320,8 @@ export default function Leaderboard({ drivers, highlightedDrivers, onDriverClick
                 }
                 const isPersonalBest = lastSecs <= personalBest + 0.0005;
 
-                // Check overall fastest (all drivers' laps 2+ up to current)
-                let overallFastest = Infinity;
-                if (isPersonalBest && lapData) {
-                  for (const [, laps] of lapData) {
-                    for (let l = 2; l <= currentLap; l++) {
-                      const t = laps.get(l);
-                      if (t) { const s = toSecs(t); if (s < overallFastest) overallFastest = s; }
-                    }
-                  }
-                }
-                const isFastest = isPersonalBest && lastSecs <= overallFastest + 0.0005;
+                // Purple: backend says this driver holds fastest lap AND their last lap equals their personal best
+                const isFastest = drv.has_fastest_lap && isPersonalBest;
 
                 const color = isFastest ? "text-purple-400" : isPersonalBest ? "text-green-400" : "text-f1-muted";