feat: add ErrorState component for failed and not-found UI

[zigzagdev]

Problem

Error and not-found states are completely unstyled across all pages:

• TopPage: bare red text + <button className="underline">
• DetailPage: naked <div> with no layout, no styling at all
• SearchResults: tiny red text squeezed into the sticky header bar

Proposed solution

Create shared/uis/ErrorState.tsx — centered card with icon, title, optional description, and optional action button — and apply it to:

• top-page-container.tsx (error → ErrorState with Retry)
• world-heritage-detail-container.tsx (error / not-found → ErrorState with Back to list)
• search-heritage-result-container.tsx (error / invalid response → ErrorState rendered standalone, not inside SearchResultsPage)

Once error states are moved to the container level, the errorMessage prop on SearchResultsPage can be removed.