Part of NL-482

Proposed Changes

• Add FailedImagesMessage component to the Substack import conversion summary screen
• Shows when backend reports failedImagesCount > 0 in the import data
• Explains that Substack did not include images in the export
• Provides numbered steps to manually save and re-upload images, with links to Media Library and Image block support docs
• Uses _n for proper singular/plural handling
• Adds SCSS for the ordered list, using logical CSS properties (padding-inline-start)

Why are these changes being made?

When importing from Substack, ~33% of images fail because Substack hosts them in protected storage that denies external access. The importer already surfaces unsupported file types and unconverted content, but silently drops images that fail with HTTP errors. Users see broken images with no explanation or guidance on how to fix them.

This PR adds actionable guidance so users know why images are missing and exactly how to fix them.

Note: This PR is safely inert until a corresponding backend change ships to populate failedImagesCount in the API response. Until then, the field defaults to 0 and the component does not render.

Testing Instructions

1. Import a Substack export at /import/{site}/newsletter/substack
2. Verify the conversion summary looks identical to before (no failedImagesCount from backend yet)
3. To test the new component, use browser DevTools to inject mock data into the React component props:
   • Set failedImagesCount = 5 → verify message appears with "5 images could not be imported..."
   • Set failedImagesCount = 1 → verify singular "1 image could not be imported..."
   • Set failedImagesCount = 0 → verify no message appears
4. Verify "Media Library" and "Image block" links point to correct support docs
5. Verify existing unsupported file types and unconverted content messages still render

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?