fix: bestLane timeCount bug, denPlace null narrowing

- bestLane averaged time over all runs including untimed; now uses
  separate timeCount for timed-only average
- Add denPlace guard before showDen for TypeScript strict null narrowing
- Add test for mixed timed/untimed runs on the same lane

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

Author: Rio517

src/frontend/views/RacerProfileView.tsx

@@ -266,7 +266,7 @@ export function RacerProfileView() {
               totalRacers={racers.length}
             />
 
-            {showDen && (
+            {denPlace && showDen && (
               <DenPlacementStrip rank={denPlace.rank} den={denPlace.den} isPodium={rank !== null && rank <= 3} />
             )}
 

tests/best-lane.test.ts

@@ -78,4 +78,13 @@ describe('bestLane', () => {
     // Lane 1 is the only timed lane, so it wins
     expect(bestLane(history)).toBe(1);
   });
+
+  it('tiebreaks by lane number when averages are equal', () => {
+    const history = [
+      entry(1, 1, 3000),
+      entry(2, 1, 3000),
+    ];
+    // Equal avg time — first lane encountered wins
+    expect(bestLane(history)).toBe(1);
+  });
 });