docs(indexeddb): add comprehensive concept page with tests

- Complete IndexedDB documentation covering database operations, transactions,
  object stores, indexes, CRUD operations, cursors, and Promise wrappers
- Add 22 tests covering utility functions and patterns
- Include filing cabinet analogy with ASCII diagrams
- Add storage comparison table (localStorage vs IndexedDB vs cookies)
- Cover real-world patterns: sync queue, database helper class
- Document common mistakes with fixes
- SEO optimized with 93% score (28/30)