fix: share auth for download url endpoint

[tommy141x]

🎯 Please make sure you are opening this Pull Request against the next branch!

📝 Description

It seems during my implementation of folders I had added preValidation to the file download endpoint - which did not exist before. This is causing 401 errors sometimes when attempting to download files from public shares.

This PR adds proper authentication to the download endpoint with support for public shares, protected shares, and private files with user authentication.

File in public share? → Yes → ✅ Allow
File in password-protected share + correct password? → Yes → ✅ Allow
Authenticated user owns file? → Yes → ✅ Allow
No valid access → ❌ Deny

🤖 Use of Artificial Intelligence (AI)

The use of AI tools is absolutely welcome and not an issue. For transparency and continuous improvement, please answer the following:

• Did you use any AI tools (such as GitHub Copilot, ChatGPT, etc.) to help develop this PR?
  • No, this PR was developed without the assistance of AI tools.
  • Yes, AI tools assisted in the development of this PR (please specify which ones and how they were used):
    • Tool(s) used: GitHub Copilot
    • Brief description of how AI contributed: Provided inline suggestions & code auto completion
• Was this PR generated entirely by an AI tool (i.e., with minimal human intervention)?
  • No
  • Yes (please provide details):

🧪 How Has This Been Tested?

Tested downloading files and folders from various pages in the dev environment and production build on my local machine.

🔄 Types of Changes

Check the relevant option(s) below:

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)
• 📚 Documentation update

✅ Checklist

• My code follows the code style of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• I have rebased and/or merged on top of the latest next branch

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🙏 Thank you for your contribution!