fix(web): prevent NaN% display in competition gap calculation (#633) (#670)

**Problem:**
Competition page shows "NaN%" for gap difference when trader P&L
percentages are null/undefined.

**Root Cause:**
Line 227: `const gap = trader.total_pnl_pct - opponent.total_pnl_pct`
- If either value is `undefined` or `null`, result is `NaN`
- Display shows "领先 NaN%" or "落后 NaN%"

**Solution:**
Add null coalescing to default undefined/null values to 0:
```typescript
const gap = (trader.total_pnl_pct ?? 0) - (opponent.total_pnl_pct ?? 0)
```

**Impact:**
- ✅ Gap calculation returns 0 when data is missing (shows 0.00%)
- ✅ Prevents confusing "NaN%" display
- ✅ Graceful degradation for incomplete data

Fixes #633

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

Co-authored-by: Claude <noreply@anthropic.com>

web/src/components/CompetitionPage.tsx

@@ -392,7 +392,7 @@ export function CompetitionPage() {
             {sortedTraders.map((trader, index) => {
               const isWinning = index === 0
               const opponent = sortedTraders[1 - index]
-              const gap = trader.total_pnl_pct - opponent.total_pnl_pct
+              const gap = (trader.total_pnl_pct ?? 0) - (opponent.total_pnl_pct ?? 0)
 
               return (
                 <div