Hide audio thumbnail from screen readers

[rushikeshmore]

Fixes

Fixes #497 by @dhruvkb

Description

The audio thumbnail component (VAudioThumbnail) previously used a title attribute on the wrapper and a dynamic alt text on the <img> element. This was misleading because:

• The generative SVG fallback (dot pattern) is not actual cover art, but the title read "Cover art for ..."
• The thumbnail is purely decorative and provides no useful information to screen reader users
• The title/alt text duplicated information already available in the audio track metadata nearby

Following the approach agreed upon in the issue discussion, this PR:

1. Adds aria-hidden="true" to the thumbnail wrapper <div>, hiding the entire element from the accessibility tree
2. Removes the :title binding from the wrapper
3. Sets alt="" on the <img> element (empty alt for decorative images per WCAG)
4. Removes the unused helpText computed property and useI18n import

Testing Instructions

1. Open any audio search results page (e.g., search for "piano")
2. Inspect the audio thumbnail element in DevTools and verify aria-hidden="true" is present on the wrapper div
3. Verify the <img> element has alt=""
4. Verify there is no title attribute on the thumbnail wrapper
5. Use a screen reader (VoiceOver on macOS: Cmd+F5) and navigate through an audio result to confirm the thumbnail is skipped
6. Check that the thumbnail still renders correctly visually (both with cover art and with the generative SVG fallback)
7. Check sensitive content: the visual blur effect still works correctly

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.

Developer Certificate of Origin

Developer Certificate of Origin

I have read and agree to the Developer Certificate of Origin version 1.1.

[rushikeshmore]

Friendly nudge — this PR has been open for ~3 weeks without a review. @krysal @obulat would either of you be able to take a look when you have a moment? Happy to adjust anything if needed.