gui: add "New Sud" menu action, disable Start/Pause/Stop for an empty sud

File > New Sud loads an empty schedule (zero steps) onto the server,
same mechanism as Load but with a built-in empty document. Tracks
whether the loaded schedule actually has steps (sud_empty) separately
from whether a Sud is configured at all (sud_loaded) - Start/Pause/Stop
now require both, and the Automatic tab's forecast shows "No sud
loaded" instead of an empty plot while sud_empty.

client/main_window.py

@@ -1255,10 +1255,13 @@ class Ui_MainWindow(object):
         self.actionPause.setObjectName("actionPause")
         self.actionStop = QtWidgets.QAction(MainWindow)
         self.actionStop.setObjectName("actionStop")
+        self.actionSudNew = QtWidgets.QAction(MainWindow)
+        self.actionSudNew.setObjectName("actionSudNew")
         self.actionSudSave = QtWidgets.QAction(MainWindow)
         self.actionSudSave.setObjectName("actionSudSave")
         self.actionSudLoad = QtWidgets.QAction(MainWindow)
         self.actionSudLoad.setObjectName("actionSudLoad")
+        self.menu_File.addAction(self.actionSudNew)
         self.menu_File.addAction(self.actionSudSave)
         self.menu_File.addAction(self.actionSudLoad)
         self.menubar.addAction(self.menu_File.menuAction())
@@ -1306,6 +1309,8 @@ class Ui_MainWindow(object):
         self.actionPause.setToolTip(_translate("MainWindow", "Pause"))
         self.actionStop.setText(_translate("MainWindow", "Stop"))
         self.actionStop.setToolTip(_translate("MainWindow", "Stop"))
+        self.actionSudNew.setText(_translate("MainWindow", "&New Sud"))
+        self.actionSudNew.setToolTip(_translate("MainWindow", "Replace the running Sud\'s schedule with an empty one"))
         self.actionSudSave.setText(_translate("MainWindow", "&Save Sud..."))
         self.actionSudSave.setToolTip(_translate("MainWindow", "Save the running Sud\'s schedule to a local sud.json"))
         self.actionSudLoad.setText(_translate("MainWindow", "&Load Sud..."))