fix: provide canDownload helper for shares and use it where appropriate

[susnux]

Summary

Use consistent logic for checking permissions of shares

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[susnux]

/backport to stable32

[susnux]

/backport to stable33

[szaimen]

🐘

[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/59008/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 08495bfc 7f8f86ab

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/59008/stable33

Error: Failed to push branch backport/59008/stable33: remote: Invalid username or token. Password authentication is not supported for Git operations.
fatal: Authentication failed for 'https://github.com/nextcloud/server.git/'

---

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

[susnux]

/backport to stable31

[susnux]

/backport to stable33