Fix ProgressWindow modality to prevent parallel operations

[MarcusStill]

During a point of sale transaction, the ProgressWindow shows the current operation status asynchronously. Since this window was previously set only to 'always on top' but not modal, users could still interact with the rest of the application (like opening a new sale). This parallel interaction could corrupt the shared state (e.g. system dictionary), leading to errors later in the cash register processing.

This commit updates ProgressWindow to call self.setModal(True) during initialization. In PySide6, this defaults the modality to Qt.ApplicationModal, preventing interactions with all other application windows and successfully protecting the sale state from unintended interference until the transaction ends.

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PR created automatically by Jules for task 1911783112662924567 started by @MarcusStill

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