fix: prevent file upload input from receiving keyboard tab focus

[kristian-zendato]

Related issue: nextcloud/server#62248

Summary

This PR updates the file upload input to prevent it from receiving keyboard focus through the Tab key.

Changes

Update the class of the file input fromhidden-visually to hidden.

Reason

Using display: none ensures the element is excluded from the tab order, preventing users from navigating to a non-interactive control while preserving the existing file upload behavior.

🤖 AI (if applicable)

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 36.30%. Comparing base (fa56e3c) to head (6345e56).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2138   +/-   ##
=======================================
  Coverage   36.30%   36.30%           
=======================================
  Files          17       17           
  Lines         774      774           
  Branches      145      145           
=======================================
  Hits          281      281           
  Misses        480      480           
  Partials       13       13           

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[susnux]

Where does this come from? hidden-visually is a server provided public CSS class, but hidden?
Sounds more like the proper fix would be

Suggested change

	class="hidden"
	class="hidden-visually"
	tabindex="-1"
	aria-hidden="true"

[susnux]

Ah its also defined in server, but the specificity is pretty low so testing did not work.
I still think it makes sense to mark this properly as hidden from the accessibility tree and remove tab sequence explicitly.