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/App.vue

@@ -1,5 +1,6 @@
 <script setup>
 import { computed, onMounted, onUnmounted, ref } from 'vue'
+import { aggregateStatus } from './aggregate'
 import * as api from './api'
 import BacktestPanel from './components/BacktestPanel.vue'
 import EquityChart from './components/EquityChart.vue'
@@ -44,6 +45,8 @@ const runningLabel = computed(() => {
   return `Running: ${list.length} strategies`
 })
 
+const displayStatus = computed(() => aggregateStatus(status.value, multiStatus.value))
+
 const scopeLabel = computed(() => {
   const list = multiStatus.value.strategies || []
   if (!list.length) {
@@ -298,10 +301,10 @@ onUnmounted(() => {
     </header>
 
     <div
-      v-if="status.halted"
+      v-if="displayStatus.halted"
       class="rounded-lg border border-rose-500 bg-rose-500/20 p-3 text-sm font-semibold text-rose-100"
     >
-      HALTED — {{ status.halt_reason }}. Trading is stopped and any open position was
+      HALTED — {{ displayStatus.halt_reason }}. Trading is stopped and any open position was
       flattened. Restart the bot to resume (limits reset).
     </div>
 
@@ -319,7 +322,7 @@ onUnmounted(() => {
       {{ status.error }}
     </div>
 
-    <StatusCards :status="status" />
+    <StatusCards :status="displayStatus" />
 
     <MultiStrategyPanel
       :strategies="strategies"