Add custom alt text support for gallery items

[Fellan-91]

Summary

This PR adds custom alt text support for gallery items in the image-based content source.

Users can now edit alt text directly in the gallery item settings without modifying the original attachment meta in the WordPress Media Library.

What changed

• added a per-item Alt field to Media Settings
• initialized item-level alt from attachment alt when no local alt exists yet
• made item-level alt the primary source for Image Alt source resolution
• applied item-level alt priority to frontend image rendering
• applied item-level alt priority to popup image data
• updated sitemap alt resolution to use:
  • item-level alt
  • description fallback for legacy behavior
  • attachment alt
• regenerated committed Gutenberg build artifacts

Tests

E2E

• added coverage for gallery item alt behavior in block flow
• added coverage for gallery item alt behavior in saved layout flow

Unit

• added coverage for:
  • gallery alt sanitization
  • image alt source resolution
  • popup alt priority
  • sitemap alt priority

Validation

• npm run build
• npm run test:unit:php -- --filter 'Test_Class_(Security_Gallery_Alt|Get_Portfolio_Image_Alt|Sitemap_Image_Alt)'
• npm run test:e2e -- tests/e2e/specs/added-images-to-block.spec.js tests/e2e/specs/added-images-to-saved-layout.spec.js