fix(files): validate dropped filenames before upload

[hamza221]

Summary

When uploading a file with a forbidden name test\ as an example it behaves differently depending on the upload method:

• Drag and drop -> reaches DAV and you get no concise user facing error

<?xml version="1.0" encoding="utf-8"?>
<d:error xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns" xmlns:o="http://owncloud.org/ns">
  <s:exception>OCA\DAV\Connector\Sabre\Exception\InvalidPath</s:exception>
  <s:message>"\" is not allowed inside a file or folder name.</s:message>
  <o:retry xmlns:o="o:">false</o:retry>
  <o:reason xmlns:o="o:">"\" is not allowed inside a file or folder name.</o:reason>
</d:error>

• Through the upload button it gets renamed

For the sake of simplicity added a check before upload on drag and drop with error handling

TODO

• ...

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
  Assisted-by: Codex:gpt-5.6-sol

[hamza221]

/backport 53e8968 to stable35

[hamza221]

/backport 53e8968 to stable34

[hamza221]

/backport 53e8968 to stable33

[hamza221]

/compile

[susnux]

This should rather give the user a change to rename it to something valid