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/brewpi_gui.py

@@ -160,6 +160,15 @@ class SudForecastPlot(FigureCanvasQTAgg):
 		self.figure.tight_layout()
 		self.draw_idle()
 
+	def show_no_schedule(self):
+		self.line.set_data([], [])
+		self.clear_progress()
+		self.ax.relim()
+		self.ax.autoscale_view()
+		self.ax.set_title("No sud loaded", fontsize='small')
+		self.figure.tight_layout()
+		self.draw_idle()
+
 	def set_progress(self, t_min):
 		self.progress_line.set_xdata([t_min, t_min])
 		self.progress_line.set_visible(True)
@@ -189,6 +198,11 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 		self.ws_client = WsClient(listener=self.msg_dispatch, loop=loop)
 		self.connected = False
 		self.sud_loaded = False
+		# Whether the loaded schedule has zero steps (e.g. just after "New
+		# Sud") - distinct from sud_loaded, which just means a Sud is
+		# configured server-side at all. Defaults True so Start/Pause/Stop
+		# stay disabled until a non-empty schedule is actually confirmed.
+		self.sud_empty = True
 		self.sud_state = None
 		self.sud_user_message = None
 		# Global, not Sud-specific - from the 'System' channel's one-time
@@ -244,6 +258,7 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 		self.actionStart.triggered.connect(self.on_action_sud_start)
 		self.actionPause.triggered.connect(self.on_action_sud_pause)
 		self.actionStop.triggered.connect(self.on_action_sud_stop)
+		self.actionSudNew.triggered.connect(self.on_action_sud_new)
 		self.actionSudSave.triggered.connect(self.on_action_sud_save)
 		self.actionSudLoad.triggered.connect(self.on_action_sud_load)
 		self.Slider_pwr_soll.valueChanged.connect(self.on_slider_pwr_soll_changed)
@@ -341,6 +356,7 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 		if not connected:
 			# Nothing reported over a dead connection can be trusted anymore.
 			self.sud_loaded = False
+			self.sud_empty = True
 			self.sud_state = None
 			self.sud_user_message = None
 			self.ambient_temp = None
@@ -353,11 +369,12 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 			self.update_status_env_label()
 
 	def update_sud_actions(self):
-		running = self.sud_loaded and self.sud_state in SUD_RUNNING_STATES
-		paused = self.sud_loaded and self.sud_state == SUD_PAUSED_STATE
+		can_run = self.sud_loaded and not self.sud_empty
+		running = can_run and self.sud_state in SUD_RUNNING_STATES
+		paused = can_run and self.sud_state == SUD_PAUSED_STATE
 		# Start also doubles as Resume while paused; Stop can still abort a
 		# paused run, but there's nothing left to Pause once already paused.
-		self.actionStart.setEnabled(self.sud_loaded and not running)
+		self.actionStart.setEnabled(can_run and not running)
 		self.actionPause.setEnabled(running)
 		self.actionStop.setEnabled(running or paused)
 
@@ -429,6 +446,11 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 		QtWidgets.QMessageBox.information(self, "Sud", message)
 		self.msg_sud.send({'Confirm': True})
 
+	def on_action_sud_new(self):
+		# Replaces the running Sud's schedule with an empty one, same as
+		# Load but with a built-in empty document instead of a file dialog.
+		self.msg_sud.send({'Load': {'Name': '', 'Description': '', 'pot_mass': 0, 'pot_material': None, 'steps': []}})
+
 	def on_action_sud_save(self):
 		# Pick the destination up front, on the GUI thread - the actual
 		# write happens in on_sud_changed(), which runs off a worker thread
@@ -579,6 +601,11 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 			_, _, schedule, _, _ = Sud._parse_data(doc)
 		except (KeyError, TypeError):
 			return
+		self.sud_empty = len(schedule) == 0
+		self.update_sud_actions()
+		if self.sud_empty:
+			self.forecast_plot.show_no_schedule()
+			return
 		# No measured starting temperature is available before a brew has
 		# started; fall back to a plausible ambient guess.
 		start_theta = self.plot_temp_ist if self.plot_temp_ist else 20.0