feat(sort): implement shell sort

[gian-gg]

Description

This PR adds full Shell Sort support to the sorting visualizer.

Changes included:

• Added ShellSortPage with step-by-step Shell Sort animation using a shrinking gap
  sequence (n/2 ... 1).
• Added Shell Sort route handling in src/App.tsx.
• Added Shell Sort metadata entry in src/algorithms/index.ts and marked it as
  implemented.
• Corrected the Shell Sort complexity label to: Best O(n log n), average ~O(n^1.5), worst O(n^2).

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not
  work as expected)
• Documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have checked that my commit message follows conventional commits

Preview

Screen.Recording.2026-02-15.at.18.47.24.mov