Windows: Fix blank window in RDP sessions

[hluk]

Clear the window display affinity before showing the warning dialog
so the window is visible during the dialog's nested event loop.
Remove the Settings-based persistence of the warning acknowledgment.

Fixes #3621

Assisted-by: Claude (Anthropic)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.33%. Comparing base (ef6e5db) to head (6fbc454).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3624      +/-   ##
==========================================
- Coverage   74.43%   74.33%   -0.11%     
==========================================
  Files         253      253              
  Lines       37916    37916              
  Branches     5120     5115       -5     
==========================================
- Hits        28222    28184      -38     
- Misses       9694     9732      +38     

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[Copilot]

Pull request overview

This PR addresses a Windows-specific usability issue in Remote Desktop (RDP) sessions where CopyQ can become effectively unusable because the modal “screen capture prevention unavailable” warning dialog is not visible to the user (yet still blocks interaction).

Changes:

• Replaces a QMessageBox::warning() call with an explicitly constructed QMessageBox.
• Attempts to ensure the warning dialog is visible in RDP by explicitly clearing display affinity (WDA_NONE) on the dialog’s native window handle before executing it.

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