fix: Handle undefined value in CostTrendChart formatter

The recharts Tooltip formatter can receive undefined values. Added
runtime type check to handle this case properly.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: silversurfer562

website/components/telemetry/CostTrendChart.tsx

@@ -74,7 +74,7 @@ export default function CostTrendChart({ data, height = 300 }: CostTrendChartPro
               borderRadius: '4px',
               padding: '8px',
             }}
-            formatter={(value: number) => `$${value.toFixed(4)}`}
+            formatter={(value) => typeof value === 'number' ? `$${value.toFixed(4)}` : value}
           />
           <Legend />
           <Line