GUI: add a spinbox to pre-dial the simulated Pot's reset temperature

btn_pot_reset always reset the simulated Pot to the ambient
temperature. Add doubleSpinBox_pot_temp next to it (shown/hidden
together, same as the button) so the user can dial in a different
starting temperature before committing it - it seeds from the same
remembered ambient value on startup but is otherwise independent and
not persisted.

tasks/pot.py's PotTask.recv() now resets the plant to whatever
temperature the 'Reset' message carries, falling back to ambient for
the bare True a caller might still send.

tasks/pot.py

@@ -22,7 +22,12 @@ class PotTask(ATask):
 	async def recv(self, data):
 		for pair in data.items():
 			if 'Reset' in pair[0]:
-				self.plant.initial(self.plant.theta_amb)
+				# The GUI's pot-temp spinbox sends the temperature to reset
+				# to directly - fall back to ambient for any older/other
+				# caller that still just sends a bare True.
+				value = pair[1]
+				temp = value if isinstance(value, (int, float)) else self.plant.theta_amb
+				self.plant.initial(temp)
 
 	async def send(self, data):
 		await self.msg_handler.send(data)