fix: blank Track Record page when the cached backtest report is pre-momentum

The momentum-sweep table read row.min_momentum_percentile.toFixed(), but a report
cached before the EV->momentum change only has min_expected_value rows. undefined
.toFixed() threw during render and — with no error boundary — blanked the whole
Track Record tab. Guard the sweep block on the new field so a stale report just
hides the sweep; re-running the backtest repopulates it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/signals/BacktestPanel.tsx

@@ -181,7 +181,10 @@ export function BacktestPanel() {
               </table>
             </div>
 
-            {report.sweep && report.sweep.length > 0 && (
+            {/* Guard on the new field so a stale cached report (pre-momentum,
+                with min_expected_value rows) hides the sweep instead of crashing
+                the whole page. Re-running the backtest repopulates it. */}
+            {report.sweep && report.sweep.length > 0 && report.sweep[0].min_momentum_percentile != null && (
               <div>
                 <p className="mb-2 text-xs font-medium uppercase tracking-widest text-gray-500">
                   Momentum-percentile sweep