🎨 Palette: Add skip to main content link for keyboard navigation

[ELO2026X]

💡 What: Added a "Skip to main content" link for keyboard accessibility.
🎯 Why: Allows keyboard-only users and screen-reader users to bypass the sidebar navigation and jump directly to the 3D model view.
📸 Before/After: Visually unchanged for mouse users. For keyboard users, a "Skip to main content" button appears in the top left when they first press Tab.
♿ Accessibility: Significant improvement for WCAG 2.4.1 Bypass Blocks. Uses sr-only focus-visible:not-sr-only to keep the UI clean until focus is needed, and tabIndex={-1} focus:outline-none on the target to manage focus semantically without visual artifacts.

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PR created automatically by Jules for task 3295766313044012065 started by @ELO2026X

[google-labs-jules]

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