feat(web): trade detail modal with prices, fees, timestamps

- Click any trade row in history table to open detail modal
- Modal shows 6 sections: 概览, 时间, 价差, 手续费, 多仓, 空仓
- Entries and exits displayed with 6 decimal precision
- Fee entry/exit and total fee displayed
- Open/close timestamps with full date-time format
- Duration, scale count, total amount, exit reason
- Orders sub-table if available
- Escape key and overlay click to close

db/trade_repo.go

@@ -206,3 +206,20 @@ func scanTrades(rows *sql.Rows) ([]TradeRecord, error) {
 	}
 	return trades, rows.Err()
 }
+
+// GetClosedStats returns convergence counts from the database.
+func (d *DB) GetClosedStats() (converged, diverged, flat, total int, err error) {
+	if err = d.QueryRow("SELECT COUNT(*) FROM trades WHERE status='closed'").Scan(&total); err != nil {
+		return
+	}
+	if err = d.QueryRow("SELECT COUNT(*) FROM trades WHERE status='closed' AND convergence='价差收敛'").Scan(&converged); err != nil {
+		return
+	}
+	if err = d.QueryRow("SELECT COUNT(*) FROM trades WHERE status='closed' AND convergence='价差发散'").Scan(&diverged); err != nil {
+		return
+	}
+	if err = d.QueryRow("SELECT COUNT(*) FROM trades WHERE status='closed' AND (convergence IS NULL OR convergence NOT IN ('价差收敛','价差发散'))").Scan(&flat); err != nil {
+		return
+	}
+	return
+}