Fix: Improve frontend code quality and null safety

- Remove unused variables and imports
- Add null/undefined safety checks for account data display
- Use optional chaining and nullish coalescing for safer data access
- Remove unused "Input Prompt" display section (keeping CoT trace)
- Fix TypeScript warnings and improve type safety

Changes:
- App.tsx: Add null checks for account fields, remove unused stats prop and input prompt display
- ComparisonChart.tsx: Remove unused imports and variables

This improves code quality without affecting functionality.

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

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

web/src/components/ComparisonChart.tsx

@@ -1,4 +1,4 @@
-import { useState, useEffect, useMemo } from 'react';
+import { useMemo } from 'react';
 import {
   LineChart,
   Line,
@@ -275,7 +275,7 @@ export function ComparisonChart({ traders }: ComparisonChartProps) {
             }}
           />
 
-          {traders.map((trader, index) => (
+          {traders.map((trader) => (
             <Line
               key={trader.trader_id}
               type="monotone"