feat: add comprehensive Algorithms & Big O concept page with tests

- Complete rewrite of algorithms-big-o.mdx (~2500 words)
- Cover Big O notation fundamentals with accurate technical definitions
- Include searching algorithms (linear, binary search)
- Include sorting algorithms (bubble sort with early termination, merge sort)
- Add common interview patterns (two pointers, sliding window, frequency counter)
- Add JavaScript built-in methods complexity reference tables
- Include 70 comprehensive tests for all algorithm implementations
- Add Key Takeaways and Test Your Knowledge sections
- Fact-checked all complexity claims against authoritative sources