Fix empty array fallback defeating React.memo optimization

[Copilot]

The inline empty array fallback performanceMap[validator.address] || [] creates a new array instance on every render when the key is undefined, causing unnecessary re-renders despite React.memo on the Item component.

Changes

• Define EMPTY_ERA_POINTS constant outside component for stable reference
• Replace inline || [] with ?? EMPTY_ERA_POINTS

// Before: new array instance on each render
eraPoints={performanceMap[validator.address] || []}

// After: stable reference
const EMPTY_ERA_POINTS: ValidatorEraPoints[] = []
// ...
eraPoints={performanceMap[validator.address] ?? EMPTY_ERA_POINTS}

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