Add E2E tests with Playwright for docs site

[yuribodo]

Description

The documentation site has pages for all 10 charts and an examples section. No integration-level testing exists to catch regressions that unit tests miss.

Test scenarios

• Docs site builds and serves without errors
• Each chart documentation page renders its interactive example
• Sales dashboard example loads completely
• Dark/light mode toggle works
• Mobile responsive layout functions correctly
• Navigation between pages works
• Code snippets are syntax-highlighted
• Chart interactions (hover, click) work in browser

Tasks

• Install and configure Playwright
• Create test suite for docs site
• Add to CI pipeline
• Configure screenshot comparison for visual regression

Why this matters

Catches integration-level regressions (broken imports, CSS issues, hydration errors) that unit tests cannot detect.