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

[mejo-]

Thanks, good finding!

[mejo-]

/backport to stable34

[mejo-]

/backport to stable33

[mejo-]

/backport to stable32

[backportbot]

The backport to stable32 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable32
git pull origin stable32

# Create the new backport branch
git checkout -b backport/8924/stable32

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 293206f4

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/8924/stable32

Error: Failed to check for changes with origin/stable32: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[backportbot]

The backport to stable33 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable33
git pull origin stable33

# Create the new backport branch
git checkout -b backport/8924/stable33

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 293206f4

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/8924/stable33

Error: Failed to check for changes with origin/stable33: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[backportbot]

The backport to stable34 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable34
git pull origin stable34

# Create the new backport branch
git checkout -b backport/8924/stable34

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 293206f4

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/8924/stable34

Error: Failed to check for changes with origin/stable34: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[mejo-]

Manual backports starting at #8946