add backtest report UI to the Track Record tab

New BacktestPanel: shows qualified hit-rate/expectancy vs the all-setups baseline,
a by-direction breakdown, and the probability calibration table (predicted vs
realized, over-confident buckets flagged amber). Includes a "Run backtest" button
that triggers the job and a plain explanation of the method and its limits.

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

frontend/src/hooks/useMarketRegime.ts

@@ -8,3 +8,11 @@ export function useMarketRegime() {
     staleTime: 60_000,
   });
 }
+
+export function useBacktestReport() {
+  return useQuery({
+    queryKey: ['backtest-report'],
+    queryFn: () => marketApi.getBacktestReport(),
+    staleTime: 60_000,
+  });
+}