fix(frontend): Add index signature to PieDataPoint interface.

Resolves TypeScript error where PieDataPoint[] was not assignable to
ChartDataInput[] by adding the required string index signature.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

frontend/src/components/analytics/TransactionDistribution.tsx

@@ -17,6 +17,7 @@ interface PieDataPoint {
   name: string;
   value: number;
   color: string;
+  [key: string]: string | number;
 }
 
 interface TooltipProps {