🎨 Palette: Add skip to main content link for accessibility

[0m364]

💡 What: Added a "Skip to main content" link to the top of all main HTML pages (index.html, products.html, concepts.html, contact.html).
🎯 Why: To allow keyboard and screen reader users to bypass repetitive navigation elements and jump directly to the primary content of the page, improving overall efficiency and usability.
📸 Before/After: See the attached visual verification media.
♿ Accessibility: This directly addresses WCAG 2.4.1 (Bypass Blocks). The link is visually hidden by default to avoid disrupting the visual design for mouse users, but slides into view when it receives keyboard focus. Additionally, tabindex="-1" was added to the target <main> containers to ensure programmatic focus is correctly shifted.

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PR created automatically by Jules for task 7565512970435375636 started by @0m364

[google-labs-jules]

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