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

[ELO2026X]

💡 What: Added a "Skip to main content" link that becomes visible when focused, allowing keyboard users to bypass the sidebar navigation and jump directly to the 3D canvas and effect details.

🎯 Why: Keyboard users and screen reader users often have to tab through long lists of navigation links (like the effects sidebar) on every page load or state change. A skip link is a standard accessibility best practice that significantly improves their navigation experience.

📸 Before/After:

• Before: Users pressing "Tab" from the start of the document would have to cycle through every effect in the sidebar before reaching the custom upload button or the 3D view controls.
• After: The very first "Tab" press reveals a "Skip to main content" link in the top left corner. Pressing "Enter" immediately moves programmatic focus to the <main> content area.

♿ Accessibility:

• Added a visually hidden anchor link (sr-only) that becomes visible on keyboard focus (focus-visible:not-sr-only, focus-visible:absolute).
• Updated ContentView's root div to a semantic <main id="main-content">.
• Added tabIndex={-1} to <main> so it can receive programmatic focus without showing a persistent focus ring (focus:outline-none).

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PR created automatically by Jules for task 15226835162471026410 started by @ELO2026X

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