Replace dual HTML5/Touch backend logic with a single TouchBackend configured to
handle both mouse and touch events, ensuring consistent drag-and-drop behavior
across all devices. Introduce a custom DocumentDragLayer to render drag previews
universally, as the touch backend does not provide a native drag image.
Update document tile, gallery, and list views to suppress native long-press
previews on iOS and prevent accidental document opening after drag actions.
Revise drag-and-drop test helpers to simulate pointer gestures compatible with
the new backend, improving reliability of automated tests.
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