Add an enable/disable switch for the temperature controller

New TempControllerBase.enabled (default False) forces y=0 in
process_pid() when off, same mechanism as the existing cooling
override - the controller tries not to drive the heater at all.

tasks/tempctrl.py: new 'Enable' recv command and 'Enabled' broadcast
on the TempCtrl channel. tasks/sud.py: SudTask now enables the
controller on any non-IDLE/DONE Sud state and disables it (forcing
power back to 0) on IDLE/DONE, so automatic mode owns it for the
duration of a run and hands back manual control once it stops/finishes.

gui: new "Enabled" checkbox on the Manual tab's Controller group,
synced from the server; the temp/heatrate setpoint controls are
disabled whenever the controller itself is disabled.

tasks/sud.py

@@ -96,6 +96,14 @@ class SudTask(ATask):
 		if value in (SudState.DONE, SudState.IDLE):
 			self.stirrer.set_duty_cycle(1.0)
 			self.stirrer.set_speed(0)
+			# A finished/stopped run no longer owns the controller - hand
+			# control back to manual mode (off by default there too).
+			self.tc.set_enabled(False)
+		else:
+			# Any other state (RAMPING/HOLDING/WAIT_USER/PAUSED) means a
+			# run is in progress and needs the controller actively driving
+			# the heater.
+			self.tc.set_enabled(True)
 
 	def on_user_message_changed(self, value):
 		asyncio.create_task(self.send({'UserMessage': value}))