Improve thumbnail for IIIF Viewer

[rayondemiel]

Optimize IIIF thumbnail loading with size parameters. In the case of external manifests, loading may take a very long time.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• [ x ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Description of Change

• Modified iiif-viewer.js to add a createThumbnailUrl() helper function
• Automatically appends IIIF Image API size parameter !n,n (default 200) to thumbnail URLs
• Maintains backward compatibility: falls back to original URL if IIIF pattern not detected

Issues Solved

Closes #

Checklist

• I targeted one of these branches:
  • [ x ] dev/8.1.x (under development): features, bugfixes not covered below
  • dev/8.0.x (main support): regressions, crashing bugs, security issues, major bugs in new features
  • dev/7.6.x (extended support): major security issues, data loss issues
• I added a changelog in arches/releases
• I submitted a PR to arches-docs (if appropriate)
• Unit tests pass locally with my changes
• I added tests that prove my fix is effective or that my feature works
• My test fails on the target branch

Accessibility Checklist

Developer Guide

Topic	Changed	Retested
Color contrast
Form fields
Headings
Links
Keyboard
Responsive Design
HTML validation
Screen reader

Ticket Background

• Sponsored by: Centre de la Recherche sur la Conservation
• Found by: @
• Tested by: @
• Designed by: @

Further comments