Chat items render rich Panels (rounded borders + Navi/You titles), but the
base Widget.get_selection only extracts text from widgets whose render is
Text/Content -- Panels return None, so a drag-select over a message or tool
output copied nothing (the highlight showed the borders, but Ctrl+C yielded
no message content).

Override _ChatItemView.get_selection: re-render the item's renderable via
the app console at the widget width (line layout identical to the screen),
strip the outer panel border lines and the │ / │ gutters, and remap the
mouse offsets onto the clean content so a partial-line selection lands on
the right characters. Plain renderables (status, turn_meta, filesystem tool
output) have no chrome and map 1:1. Ctrl+C (Screen.copy_text) then copies
the clean text via OSC 52.