fix: add nullish coalescing for dashboard stats to prevent TypeError

Dashboard page crashed with 'Cannot read properties of undefined
(reading toLocaleString)' when agent stats fields were undefined.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gcmsg

web/app/src/pages/ProviderDashboardPage.tsx

@@ -65,17 +65,17 @@ export function ProviderDashboardPage() {
         />
         <AgentStatsCard
           title={t('provider.totalCalls')}
-          value={data.total_calls.toLocaleString()}
+          value={(data.total_calls ?? 0).toLocaleString()}
           icon={PhoneCall}
         />
         <AgentStatsCard
           title={t('provider.successRate')}
-          value={`${data.success_rate.toFixed(1)}%`}
+          value={`${(data.success_rate ?? 0).toFixed(1)}%`}
           icon={CheckCircle}
         />
         <AgentStatsCard
           title={t('provider.avgLatency')}
-          value={`${data.avg_latency_ms.toFixed(0)}ms`}
+          value={`${(data.avg_latency_ms ?? 0).toFixed(0)}ms`}
           icon={Timer}
         />
       </div>
@@ -128,13 +128,13 @@ export function ProviderDashboardPage() {
                     </Badge>
                   </TableCell>
                   <TableCell className="text-muted-foreground">
-                    {agent.total_calls.toLocaleString()}
+                    {(agent.total_calls ?? 0).toLocaleString()}
                   </TableCell>
                   <TableCell className="text-muted-foreground">
-                    {agent.success_rate.toFixed(1)}%
+                    {(agent.success_rate ?? 0).toFixed(1)}%
                   </TableCell>
                   <TableCell className="text-muted-foreground">
-                    {agent.avg_latency_ms.toFixed(0)}ms
+                    {(agent.avg_latency_ms ?? 0).toFixed(0)}ms
                   </TableCell>
                 </TableRow>
               ))}