feat(cpp): Add Shell Sort visualization to sorting module

[MJ-thunder]

🧩 Summary

Implemented Shell Sort visualization within cpp/src/algorithms/sorting.cpp to expand the set of available sorting algorithms.

🔍 Details

• Added a new shellSort() function following the same structure and visualization style used in existing algorithms (Bubble, Merge, Quick, Heap, Counting).
• Incorporated:
  • Step-by-step tracking of operations
  • Highlighted and compared indices
  • Operation count tracking
  • Time and space complexity metadata
• Verified compilation and output consistency with the current framework.

🧠 Complexity

• Time Complexity: O(n log² n)
• Space Complexity: O(1)

✅ Testing

• Successfully compiled and executed locally without runtime errors.
• Visualization steps confirmed to integrate correctly with existing UI logic.

🙌 Notes for Maintainers

This addition enhances the educational scope of the visualizer by including an advanced gap-based sorting algorithm.
Please review and merge when convenient.