Move the PlotWindow + SensorGadgetWindow create/show/sync/close lifecycle out of MainWindow into Services/SecondaryWindowCoordinator. The coordinator owns the _plotWindow and _gadgetWindow fields and exposes UpdatePlotWindowVisibility / UpdateGadgetVisibility / SyncGadgetSensors / ApplyTheme / RedrawPlot / CloseAll.
MainWindow's matching methods become one-line delegators; the gadget's HideShowMainWindowRequested event is wired back via a callback passed into the coordinator's constructor.
MainWindow shrinks from 1362 to 1300 lines; no behaviour change.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
