feat: add EmptyState component with configurable illustration and CTA

[Buchi-Einstein]

Implements a reusable EmptyState component for meaningful empty states.

• Configurable illustration (React node), title, description, primary CTA, secondary CTA
• Illustration centered above text content
• Primary CTA styled as filled Button, secondary as outline
• Minimum height (280px) prevents layout shift
• Centered layout with max-width constraint (max-w-sm)
• Configurable heading level (h1-h6, default h2)
• Illustration rendered with aria-hidden for accessibility
• Built-in illustration variants: NoCollections, NoResults, NoRecentlyPlayed, NoFollowers
• Illustrations stored in components/ui/EmptyState/

Closes #99

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[Buchi-Einstein]

@Akatenvictor please merge pr