PDF link annotations lack discernible names, causing Lighthouse accessibility warnings and poor screen reader support. (#1424)

This update adds aria-label attributes to links using overlaidText from annotation data:

1. Add setAnnotationContext/clearAnnotationContext to LinkService to store annotation data
2. Set annotation context before rendering annotation layer
3. Use overlaidText from annotations in addLinkAttributes to set aria-label

Reproduce:

1. Load test.pdf in the test page
2. Open DevTools → Elements tab
3. Select an <a> element in the annotation layer
4. View ARIA attributes in the Accessibility tab

Effect:

• Before: Links have no aria-label, causing Lighthouse warnings
  
• After: Links have aria-label with meaningful text for screen readers