feat: wrap elements in WebElement for unified API across helpers

- Import and use WebElement wrapper in lib/els.js for all element functions
- Provides consistent API (getText, getAttribute, click, etc.) across Playwright, WebDriver, Puppeteer
- Update unit tests to work with WebElement instances
- Add comprehensive element-based testing guide (docs/element-based-testing.md)
- Update docs/els.md to remove portability warning and link to new guide

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: DavertMik