Accessibility improvements: Skip links and focus management

[gcamacho079]

Description

This PR adds critical accessibility features for keyboard navigation:

New features:

• Skip link: Added "Skip to main content" link that appears on focus, allowing keyboard users to bypass navigation
• Focus reset on route changes: Focus automatically moves to a hidden span on page navigation (ignoring hash/query changes)
• Content focusability: Main content area now accepts focus via tabindex="-1"

Styling improvements:

• Added button CSS variables for consistent styling across light/dark themes
• Replaced hardcoded colors with CSS custom properties in DocSetPanel

For future developers:

• Use --button-* CSS variables for consistent button styling
• Skip link implementation follows WCAG guidelines and can be replicated elsewhere
• CSS custom properties provide theme-aware styling without hardcoded values

Related issues

N/A

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