feat(dashboard): add Cash Disponible card to metrics grid

Shows /api/summary cash_available (now consistent with the executor's cash
model) next to Capital Deployed, with its share of bankroll as subtitle and
progress bar.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

dashboard/src/App.jsx

@@ -216,6 +216,12 @@ export default function App() {
           value={fmtUSD(summary.total_deployed)}
           subtitle={`${summary.total_trades} trades`}
         />
+        <MetricCard
+          title="Cash Disponible"
+          value={fmtUSD(summary.cash_available)}
+          subtitle={`${fmtPct(summary.cash_available / summary.paper_bankroll)} del bankroll`}
+          progress={summary.cash_available / summary.paper_bankroll}
+        />
       </div>
 
       {/* Performance chart */}