Improves the operation output views in the Avalonia UI

[Gabriel Dufresne (GabrielDuf)]

#4681

• Selectable text across multiple lines. Previously each log line was rendered as its own TextBlock inside an ItemsControl, so selection could not span more than one line. Replaced with a single SelectableTextBlock populated with colored Run inlines (one per line) + LineBreaks, preserving per-line coloring (error / debug / normal) while allowing click-drag selection across the whole log and Ctrl+C copy.
• Non-modal windows. Both the live/success log (OperationOutputWindow) and the failed-operation dialog (OperationFailedDialog) were shown via ShowDialog, which blocked all interaction with the main window. Switched to Show(owner) so the windows remain owned by the main window (stay on top, close with it) but no longer block the rest of the app.

Files changed

• UniGetUI.Avalonia/Views/DialogPages/OperationOutputWindow.axaml + .axaml.cs
• UniGetUI.Avalonia/Views/DialogPages/OperationFailedDialog.axaml + .axaml.cs (also removed the now-unused OutputLineVm helper class)
• UniGetUI.Avalonia/ViewModels/DialogPages/OperationViewModel.cs (ShowDetails now uses Show instead of ShowDialog for both branches)