Return visible errors for unsupported Notion content

[cwjcwj95]

Summary

• Render a visible .notion-error message for unsupported Notion block types instead of silently returning an empty <div />.
• Show a specific Database unsupported message when a collection_view block is present but no collection data is available.
• Add styling for the error block so unsupported content is visible without breaking the surrounding page layout.

Why

Fixes #87.

Before this change, unsupported content such as a Notion database could make the page appear to not load, especially in production where the existing unsupported-type log is not visible to users. The renderer now explains which unsupported content should be removed.

Verification

• Compared patched tsc output with a baseline copy of current master; both show the same existing TypeScript environment errors:
  • missing process type under the constrained --types react,react-dom check
  • existing customBlockComponents never narrowing error
• The diff did not introduce new TypeScript errors versus baseline.

Note: a full install using the repository's old typescript@3.9 currently resolves newer @types/* packages from npm, which produce dependency syntax errors unrelated to this patch.