Add a "Pot reset to ambient temperature" button, sim-only

New btn_pot_reset next to the ambient entry, sends {"Pot": {"Reset":
true}}; tasks/pot.py's PotTask.recv() (previously a no-op) now resets
the plant to its current ambient temperature (Pot.initial()) on that
command.

The button only appears when the server reports its Pot is simulated
(new "PlantSim" field on the System channel, derived from
config.json's previously-unused Controller.plant_name) - resetting a
simulated plant's temperature is meaningless for a real one.

client/brewpi.ui

@@ -3711,6 +3711,22 @@
      <string>Set the simulated/assumed ambient temperature - press Enter to apply</string>
     </property>
    </widget>
+   <widget class="QPushButton" name="btn_pot_reset">
+    <property name="geometry">
+     <rect>
+      <x>700</x>
+      <y>500</y>
+      <width>220</width>
+      <height>31</height>
+     </rect>
+    </property>
+    <property name="toolTip">
+     <string>Only available when the server's Pot is simulated</string>
+    </property>
+    <property name="text">
+     <string>Pot reset to ambient temperature</string>
+    </property>
+   </widget>
    <widget class="QLabel" name="label_8">
     <property name="geometry">
      <rect>