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/tests/test_api.py

@@ -134,6 +134,8 @@ def test_start_many_and_multi_status(runtime: Runtime, market: FakeMarketData) -
         assert status["running"] is True
         assert len(status["strategies"]) == 2
         assert status["equity"] == 10000
+        # Per-strategy fields the dashboard aggregates must be present.
+        assert {"day_pnl", "drawdown_pct"} <= set(status["strategies"][0])
 
         client.post("/api/bot/stop")