Render clear errors for unsupported Notion blocks

[ncthuc2004]

Summary

• render a visible fallback when Notion returns an unsupported block type instead of silently outputting an empty placeholder
• add a custom renderUnsupportedBlock escape hatch and showUnsupportedBlockError={false} for the previous silent behavior
• surface missing or unsupported collection_view metadata as an unsupported block error
• document the new options and add React server-render tests

Fixes #87.

Validation

• yarn test --runInBand
• npx tsc --noEmit --skipLibCheck
• yarn lint (passes with existing warnings in asset/page-header)
• yarn build
• git diff --check
• UTF-8/mojibake check