feat: implement empty states for catalog and search/filter

[Sughter99]

Closes #9

What this PR does

Polishes the empty states to match web behavior with friendly messaging
and a clear/reset action for the no-matches case.

Changes

• Created src/components/EmptyState.tsx

  • Reusable component with title, body, icon and optional action button
  • Used for both empty catalog and no-matches scenarios

• Updated App.tsx

  • Replaced inline empty state with EmptyState component
  • Catalog empty: shows friendly guidance to connect to MindVault server
  • No matches: shows "No matches" with a "Clear search" button that
    resets the search input
  • Removed old unused emptyState/emptyTitle/emptyBody styles

Acceptance Criteria

• When catalog empty: shows friendly empty state with guidance
• When filters/search yield no matches: shows "No matches" and
  a clear/reset action

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