🎨 Palette: Make custom div buttons accessible on welcome screen

[thirdeyenation]

💡 What

Added ARIA roles, tabindex, keyboard event handlers, and focus-visible styles to the .welcome-action-card elements on the welcome screen.

🎯 Why

These elements look and act like buttons, but were marked up as regular divs. This meant keyboard users and screen reader users could not interact with them. This change makes the welcome screen fully accessible and usable for all users.

♿ Accessibility

• Added role="button" to provide semantic meaning for screen readers.
• Added tabindex="0" to make the elements focusable via keyboard navigation.
• Added @keydown.enter and @keydown.space.prevent event handlers so keyboard users can activate the actions.
• Added :focus-visible styles (matching the existing hover state + an outline) so keyboard users have clear visual feedback when an element is focused.

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PR created automatically by Jules for task 11083504222213058978 started by @thirdeyenation

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