Enhanced the UI with a "Vorschau" (Preview) button to display
PDF previews of selected envelopes. Introduced a `DxPopup`
to show envelope details, including ID and title, with a
"Schließen" (Close) button.
Replaced `SelectedDataItemChanged` with `RowClick` and
`RowDoubleClick` events for improved row selection.
Single-click toggles row selection, while double-click opens
the preview popup. Added `_previewVisible` to manage popup
visibility and implemented `ShowPreview` for button-triggered
previews. Removed `OnSelectedEnvelopeChanged` as it is now
handled by the new row events. Updated grid layout persistence
to align with the new event structure.
TekH
ec3a15a467