🎨 Palette: [UX improvement] Add keyboard accessibility to welcome action cards

[thirdeyenation]

💡 What: Added role="button", tabindex="0", and keyboard event handlers (@keydown.enter and @keydown.space.prevent) to the custom .welcome-action-card divs on the welcome screen. Updated CSS to include .welcome-action-card:focus-visible styling alongside the existing :hover styles.
🎯 Why: To make the main navigation cards on the welcome screen fully accessible and navigable using the keyboard, improving the experience for users relying on keyboard navigation and screen readers.
📸 Before/After: The action cards now receive a clear focus outline (reusing the hover state border and background) when navigating via the Tab key, and can be activated using Enter or Space.
♿ Accessibility: Fixes a critical keyboard accessibility issue by transforming non-semantic div click handlers into proper button-like interactions.

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PR created automatically by Jules for task 8073468358740675883 started by @thirdeyenation

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