Add descriptive alt text on work and file-set representative media accessibility

[Janell-Huyck]

Summary

This branch improves accessibility (including common WAVE findings) across Hyrax views and the app shell, adds meaningful alt text for representative thumbnails on work and file-set pages, and tightens the notifications badge behavior for screen readers. View and helper specs are added or updated to lock in the new markup.

Motivation

• Thumbnails on work/file-set show pages should expose useful alt text instead of generic or missing descriptions.
• Several UI patterns produced empty headings, unlabeled controls, or redundant navigation links that hurt accessibility tooling and assistive tech.
• The unread notification count badge should not announce “0” when there is nothing to report, while still showing a visible count when messages are unread.

Changes

Thumbnails and representative media

• Override Hyrax hyrax/file_sets/media_display partials for image, PDF, and office documents to pass alt: thumbnail_alt_text_for(...).
• Update hyrax/base/_representative_media.html.erb so the default representative image uses the same alt helper.

Shell and Hyrax UI accessibility

• Search: Add a proper <label> (visually hidden where appropriate) for catalog and “my” dashboard search forms.
• Work show actions: Associate a sr-only label with the batch checkbox and add aria-label for “include work in collection” / batch selection semantics.
• Parent relationships: Only render the relationships <h2> when parent works exist (_parent_relationship_table).
• Collection show (public and dashboard): Render the search-results heading only when search parameters are present, avoiding an empty <h2>.
• User menu: Improve dropdown semantics (role, aria-*, menuitem) and wire the toggle to the menu it controls.
• Toolbar: Use non-navigating dropdown toggles for “My Works” / “My Collections” so each destination appears once as a real link (Bootstrap 3–compatible).

Notifications

• Extract notification_count_badge from render_notifications in HyraxHelper.
• When the unread count is zero, render an empty badge span with aria-hidden="true" and invisible styling; when positive, show the numeric count with the danger label and no aria-hidden.

Locales

• Add English strings in config/locales/hyrax.en.yml for new user-visible copy tied to the above views.

Tests

• New specs for media display partials, representative media, show actions, and parent relationship table.
• Extended specs for toolbar, user util links (including unread notification badge behavior), search forms, collection show pages, and notification_count_badge in hyrax_helper_spec.

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Wave view of item detail page. Includes 1 warning for a duplicate link that cannot be fixed without more in-depth JavaScript rework.

[haitzlm]

Excellent work on remediation. All tests passed, who am I do doubt what they did?