fix: add contextual empty-state variants for dashboards (#54)

[rafaio1]

Fixes #54.

• Enhance EmptyState component with a variant prop (default, success, filtered, error) providing distinct visual treatments (border, background, icon color) for different empty-state contexts.
• Update maintainer dashboard to use the success variant when all PRs are reviewed ("All caught up"), distinguishing a positive empty state from a call-to-action one.
• Update contributor dashboard to use success variant for completed claims tab and default for active claims tab, with appropriate icons.
• Update sponsor dashboard to conditionally use success variant when no active bounties exist but the sponsor has previously funded (indicating completion rather than abandonment).
• TypeScript compiles cleanly with npx tsc --noEmit.

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