feat(a11y): add accessible skip to main content link

[google-labs-jules]

🎨 Palette: Added accessible Skip to Main Content link

💡 What:

• Added a "Skip to main content" link that appears when focused (via keyboard tab).
• Refactored src/pages/Index.tsx to structurally separate the <Header /> from the <main> content content.

🎯 Why:

• Allows keyboard and screen reader users to bypass the navigation menu and jump directly to the primary content.
• Improves semantic HTML structure by ensuring the header is distinct from the main content area.

📸 Before/After:

• Before: No skip link; keyboard users tab through all nav items before reaching content. Header was inside <main>.
• After: First tab stop is "Skip to main content". Activating it jumps focus to <main>. Header is sibling to <main>.

♿ Accessibility:

• WCAG 2.1 Bypass Blocks (Criterion 2.4.1) compliance.
• Improved semantic landmarks.

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PR created automatically by Jules for task 15023331386480978236 started by @Shamilzbr-bt

[google-labs-jules]

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