Fixes the accessibility issue where screen readers receive no indication when images are unavailable.

[sobreiti]

Closes

Fixes #4665

Changes Made

• Added error handling for image load failures
• Dynamic alt text that updates to "Image not available: [title]" when images fail
• Screen reader announcements using aria-live
• Visual fallback for sighted users
• Maintained high contrast and reduced motion support

Testing

• Tested with screen reader (specify which one)
• Tested with broken image URLs
• Tested visual appearance
• Tested in multiple browsers

Checklist

• My pull request has a descriptive title (not a vague title likeUpdate index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.
• I ran the DAG documentation generator (ov just catalog/generate-docs for catalog
  PRs) or the media properties generator (ov just catalog/generate-docs media-props
  for the catalog or ov just api/generate-docs for the API) where applicable.

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