feat: add Big-O time & space complexity cheat sheet page

[ashrion]

Summary

Adds a Big-O time & space complexity cheat sheet — a single revision page covering data structure operations, sorting algorithms, graph algorithms and complexity classes, so users don't have to re-google the same tables before every interview.

What changed

• New frontend/src/data/complexityCheats.js:
  • 10 data structures with average + worst-case access/search/insert/delete
  • 9 sorting algorithms (best/average/worst time, space, stability) with notes
  • 9 graph algorithms + 5 common patterns (binary search, two pointers, sliding window, hashmap, memoization)
  • 7 complexity class tiers (O(1) → O(n!)) with plain-language explanations and examples
• New frontend/src/pages/ComplexityCheats/ComplexityCheats.jsx:
  • tabbed sections (Data Structures / Sorting / Graph & Patterns / Complexity Classes)
  • live filter across names, notes and descriptions
  • sortable sorting table — click Best/Average/Worst/Space column headers to re-sort
  • average/worst-case toggle for data structures
  • color-coded complexity badges (green → red scale) and click-to-expand "why it matters" notes
• App.jsx route /complexity-cheat-sheet + Sidebar.jsx nav entry under Interview.

Architecture notes

• Pure data module + presentational page, no dependencies or backend — follows the existing data-driven page pattern.
• Complexity-to-color mapping and table sorting are isolated helpers so the data stays plain JSON-safe and easy to extend (add an algorithm → it shows up with correct badges automatically).
• Sort order for the table uses an explicit complexity rank so "O(n)" sorts before "O(n log n)" and "O(n²)".

How to test

1. cd frontend && npm run dev
2. Open /complexity-cheat-sheet (Interview > Complexity Cheat Sheet).
3. Switch tabs, type "heap" in the filter, click the "Worst" column to re-sort, toggle Average/Worst for data structures, expand a row for notes.

Verification

• vite build passes; ESLint clean on all new files.
• Fixes [Feature]: Big-O Time & Space Complexity Cheat Sheet #1026 (gssoc26)
• No backend or workflow files touched.