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.

frontend/src/aggregate.js

@@ -0,0 +1,34 @@
+/**
+ * Top-of-dashboard status. With one strategy it is that strategy's status;
+ * with several, the per-strategy numbers are aggregated for the whole account.
+ */
+export function aggregateStatus(status, multiStatus) {
+  const list = multiStatus?.strategies || []
+  if (list.length <= 1) return status
+
+  const sum = (field) => list.reduce((acc, s) => acc + (s[field] || 0), 0)
+  const halted = list.filter((s) => s.halted)
+
+  return {
+    ...status,
+    running: multiStatus.running,
+    price: multiStatus.price ?? status.price,
+    cash: sum('cash'),
+    equity: multiStatus.equity,
+    starting_balance: multiStatus.starting_balance,
+    realized_pnl: sum('realized_pnl'),
+    unrealized_pnl: sum('unrealized_pnl'),
+    total_pnl: multiStatus.total_pnl,
+    day_pnl: sum('day_pnl'),
+    // Worst individual strategy drawdown; the portfolio peak is not tracked.
+    drawdown_pct: Math.max(0, ...list.map((s) => s.drawdown_pct || 0)),
+    trades_today: sum('trades_today'),
+    open_positions: sum('open_positions'),
+    halted: halted.length > 0,
+    halt_reason:
+      halted
+        .map((s) => s.halt_reason)
+        .filter(Boolean)
+        .join('; ') || null,
+  }
+}