🎨 Palette: Enhance copy button focus visibility and error feedback

[ightevenmckane187]

This PR enhances the copy button UX on the Sovereign Cypher-Tube landing page.

💡 What

• Added high-contrast :focus-visible focus outlines for .copy-button elements against dark code container backgrounds.
• Added visual styling (border-color: var(--success) and box-shadow: 0 0 8px var(--success-glow)) for .copy-button.copied.
• Updated clipboard copy failure error handling to set copyText.textContent = 'Error!' and update aria-label to "Failed to copy command to clipboard".

🎯 Why

Low-contrast focus states hinder keyboard-only navigation. Furthermore, silent clipboard failures provide poor accessibility feedback for screen-reader users.

♿ Accessibility

• Explicit :focus-visible ring ensures clear visibility during keyboard tabbing.
• Dynamic aria-label updates communicate copy success and failure to screen readers.

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PR created automatically by Jules for task 12201474105818714398 started by @ightevenmckane187

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