Add unit tests for the document preview component falling back to a download link when the IPFS gateway times out

[Chucks1093]

Summary

The document preview fetches the invoice PDF from IPFS and renders it inline. When the gateway times out the component should stop the spinner and show a 'Preview unavailable — download directly' link instead of hanging indefinitely.

Scope

• Test a successful IPFS fetch renders the PDF preview iframe
• Test a fetch that times out after 10 seconds shows the download link fallback
• Test a 404 response shows 'Document not found' rather than the download link
• Test clicking the download link opens the raw IPFS URL in a new tab
• Assert the loading spinner is removed in all three cases

Acceptance Criteria

• Successful fetch renders preview iframe
• Timeout shows download link fallback
• 404 shows 'Document not found' message
• Download link opens raw IPFS URL in new tab
• Loading spinner removed in all cases

ETA: 12 hours

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