feat: add comprehensive Design Patterns concept page with tests

- Cover 6 practical JS patterns: Module, Singleton, Factory, Observer, Proxy, Decorator
- Add Note clarifying Module is JS-specific, not a GoF pattern
- Include ASCII diagrams, real-world analogies, and code examples
- Add common mistakes section and pattern selection guide
- Include 23 passing tests covering all patterns
- Add Key Takeaways, Test Your Knowledge Q&A, and curated resources