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

[ELO2026X]

💡 What:
Added a visually hidden "Skip to main content" link at the top of the application that becomes visible on keyboard focus. The target main container was updated to receive this focus without displaying an outline.

🎯 Why:
This is a critical accessibility feature. It allows keyboard-only users and screen-reader users to bypass the side navigation and jump directly to the primary 3D visual effects content, improving their experience significantly.

📸 Before/After:
Before: Users had to tab through the entire effect sidebar before reaching the canvas.
After: Pressing Tab immediately after page load focuses a "Skip to main content" link that instantly focuses the main area when clicked.

♿ Accessibility:

• Added skip link with correct href to target container ID.
• Used Tailwind's sr-only and focus-visible: utilities to ensure the link is only visible when it needs to be.
• Updated the content container from a <div> to a semantic <main> tag.
• Added tabIndex={-1} to <main> to allow programmatic focus without interrupting normal tab flow.
• Added focus:outline-none to <main> to prevent a confusing focus ring on the entire container when skipped to.

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PR created automatically by Jules for task 3956059875686870836 started by @ELO2026X

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