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/schemas.py

@@ -177,6 +177,8 @@ class StrategyStatusOut(BaseModel):
     halted: bool = False
     halt_reason: str | None = None
     trades_today: int = 0
+    day_pnl: float = 0.0
+    drawdown_pct: float = 0.0
     error: str | None = None