🎨 Palette: Add skip to main content link

[ELO2026X]

💡 What
Added a "Skip to main content" link to the root App component, allowing users to bypass the sidebar navigation. Also updated ContentView to act as the target by using a semantic <main> tag with id="main-content" and tabIndex={-1}.

🎯 Why
For users who rely on keyboard navigation or screen readers, having to tab through a persistent sidebar every time they want to interact with the main content area is extremely tedious. A skip link is a standard accessibility best practice to solve this problem.

📸 Before/After
Before: Keyboard users must tab through every single effect button in the sidebar before reaching the "Upload Custom Model" button or interacting with the 3D view.
After: The first tab press immediately focuses the "Skip to main content" link. Pressing Enter immediately shifts focus to the main content area, allowing the user to bypass the entire sidebar.

♿ Accessibility

• Provides a direct skip link to bypass repetitive navigation.
• Uses semantic <main> tag for the primary content area.
• Link is visually hidden using .sr-only but becomes visible when receiving focus via .focus-visible:not-sr-only to ensure sighted keyboard users can see it.
• Prevents focus outline ring from appearing around the entire main container when skipped using focus:outline-none.

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PR created automatically by Jules for task 2242009912088168453 started by @ELO2026X

[google-labs-jules]

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