feat(filters): add themed empty state for FiltersSidebar

[MJ-RWA]

Summary

This PR adds a themed empty-state experience to the FiltersSidebar when no filter data is available, improving the user experience for the GrantFox FWC26 campaign.

Changes Made

• Added a themed empty-state illustration for FiltersSidebar.
• Enhanced the EmptyState component to support the sidebar empty-state use case.
• Added a helpful call-to-action when no data is available.
• Ensured responsive behavior across supported breakpoints.
• Maintained design-token consistency and dark-mode compatibility.
• Improved accessibility to align with WCAG 2.1 AA guidelines.
• Added focused tests covering the new empty-state behavior.
• Updated documentation to reflect the new UI behavior and any exposed component changes.

Testing

• Verified the empty state is displayed when no filter data is available.
• Verified the sidebar continues to render normally when data exists.
• Verified the CTA renders and behaves as expected.
• Ran existing test suite to ensure no regressions.

Closes: #722

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[greatest0fallt1me]

CI green, LGTM ✨ merged.