fix: update ShowImageModal to use fullUrl for show original image ins…

[mosi-kha]

📝 Summary

Clicking an embedded image opened the modal on currentImage.previewUrl — the
1024x1024 preview, upscaled to an 80vh modal and visibly soft. fullUrl (the
original, same route + &preferRawImage=1) was already on the attachment object
and unused, so this just points the modal's <img> at it.

Inline rendering is untouched — documents still load previews, the original is
fetched only on an explicit click. Also matches handleAttachmentClick(), which
already uses fullUrl / the Viewer; the image modal was the only click-through
still showing a preview.

🖼️ Screenshots

🏚️ Before	🏡 After
B	A

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits
• Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests
• Documentation has been updated or is not required

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI tools
• The AI-generated content was reviewed, comprehended and tested by a human