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/api/market.ts

@@ -1,6 +1,10 @@
 import apiClient from './client';
-import type { MarketRegime } from '../lib/types';
+import type { BacktestReport, MarketRegime } from '../lib/types';
 
 export function getMarketRegime() {
   return apiClient.get<MarketRegime>('market/regime').then((r) => r.data);
 }
+
+export function getBacktestReport() {
+  return apiClient.get<BacktestReport | null>('backtest/report').then((r) => r.data);
+}