Improve render performance with React memoization and callback optimizations

[Copilot]

This PR significantly improves the render performance of the GitHub Grid Agent application by implementing comprehensive React optimization patterns.

Performance Issues Identified

The application was experiencing performance bottlenecks due to:

• Excessive re-renders of grid components when unrelated state changed
• Expensive markdown parsing operations being repeated unnecessarily
• Inefficient state update patterns causing entire object trees to be recreated
• Missing memoization on callback functions leading to dependency chain re-renders

Key Optimizations Implemented

🚀 Component Memoization

• Added React.memo to critical components: Row, Cell, GridCellContent, and TableContent
• Components now only re-render when their props actually change, preventing cascade re-renders

⚡ Callback Optimization

• Wrapped setGridState, selectRow, and updateCellState in useCallback to prevent unnecessary dependency changes
• Fixed ESLint warning about setGridState dependencies changing on every render

🔧 Efficient State Updates

• Optimized updateCellState function to:
  • Use findIndex instead of mapping over all columns
  • Include change detection to avoid updates when values are identical
  • Create minimal new objects instead of deep cloning entire state tree

📝 Expensive Operation Caching

• Memoized marked.parse() operations in SelectedRowPanel to avoid re-parsing markdown on every render
• Memoized suggestions array in Home.tsx to prevent recreation

Technical Benefits

• Reduced Re-renders: Components only update when props actually change
• Improved Memory Usage: Less object creation during frequent state updates
• Faster Rendering: Heavy operations like markdown parsing are cached
• Better UX: More responsive grid interactions, especially with large datasets

Validation

• All existing tests continue to pass (43/43)
• Added comprehensive performance tests that validate memoization effectiveness
• No breaking changes to public APIs
• Build process remains functional

The optimizations are particularly beneficial for grids with many rows/columns where state updates were previously causing expensive re-render cascades throughout the component tree.

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