Aggregate the top status cards across strategies

With several strategies the header cards showed only the first one. Expose
day_pnl and drawdown_pct per strategy and add aggregateStatus() (frontend) so the
cards show the whole account: summed cash/equity/P&L/trades/positions, worst
drawdown, and any halt. Single-strategy runs are unchanged. Adds tests.

backend/app/engine/manager.py

@@ -158,6 +158,8 @@ class MultiStrategyManager:
                     halted=engine_status.halted,
                     halt_reason=engine_status.halt_reason,
                     trades_today=engine_status.trades_today,
+                    day_pnl=engine_status.day_pnl,
+                    drawdown_pct=engine_status.drawdown_pct,
                     error=engine_status.error,
                 )
             )