feat: extend test coverage (15 new tests)

[PavelShilin89]

Summary

• Add 6 new test files with 15 new tests (total: 34 tests across 11 files)
• navigation.spec.ts — sidebar sections, link navigation, page heading and title
• anchors.spec.ts — anchor links integrity, named anchors for deep linking
• copy-button.spec.ts — code block copy button presence and click handling
• broken-links.spec.ts — sidebar links return 200, external links reachable
• page-404.spec.ts — non-existent page handling, navigation still works
• mobile-viewport.spec.ts — iPhone 12 viewport: page load, no horizontal scroll, code blocks visible
• Update README with descriptions of all new tests and updated project structure

Test plan

• All 34 tests pass locally (npx playwright test)
• CI workflow passes