docs(localstorage-sessionstorage): add comprehensive concept page with tests

- Replace placeholder with full Web Storage API documentation (~4000 words)
- Cover localStorage vs sessionStorage differences with comparison table
- Include JSON serialization patterns and gotchas
- Document storage events for cross-tab communication
- Add security considerations with OWASP reference
- Provide decision flowchart for storage selection
- Include common patterns: theme preference, form wizard, recently viewed
- Add 79 comprehensive tests covering all code examples
- Verify all external links (MDN, articles, videos)