Agent/unify responsive navigation

[Foscat]

This pull request introduces significant improvements to documentation, accessibility, and design quality assurance for the project. The main updates include a comprehensive design QA report, enhanced accessibility features in Playwright test fixtures, and clearer documentation for navigation components.

Design QA and Documentation Updates:

• Added a detailed design-qa.md report documenting visual comparisons, accessibility findings, viewport normalization, and implementation checklists for both desktop and mobile. The report confirms no critical visual mismatches remain and documents accepted deviations and fixes.
• Updated docs/navigation.md to clarify the architecture and behavior of navigation components:
  • Improved descriptions for StickyNav to highlight viewport-independent headers and consolidated utilities.
  • Updated StickySectionNav to emphasize icon-triggered section navigation and clarified the distinction between right-drawer discovery and scroll-spy coordination. [1] [2]

Accessibility and Test Fixture Enhancements:

• Added skip-link, loading indicator, and "Back to top" button to Playwright page fixtures for better accessibility coverage and UI feedback during automated tests. [1] [2] [3]

These changes strengthen the project's documentation, accessibility, and design reliability, ensuring both the implementation and testing environments reflect the intended user experience.