Add a real-time Plot tab to brewpi_gui, mirroring demo_sud figure 1

Embeds a dark-themed matplotlib canvas (3 stacked strip charts: theta
ist/soll, heatrate ist/soll, heater power_set/power_eff) in a new Plot
tab, sampled once per second from the latest websocket-received values.

HeaterTask now also emits a PowerSet message (the pre-discretization
target power) alongside the existing Power (effective) message, since
the GUI had no way to see the set-point side of figure 1's bottom plot
otherwise.

client/brewpi.ui

@@ -708,8 +708,20 @@
     </widget>
     <widget class="QWidget" name="tab_2">
      <attribute name="title">
-      <string>Seite</string>
+      <string>Plot</string>
      </attribute>
+     <layout class="QVBoxLayout" name="verticalLayout_plot">
+      <item>
+       <widget class="QWidget" name="widget_plot" native="true">
+        <property name="minimumSize">
+         <size>
+          <width>0</width>
+          <height>400</height>
+         </size>
+        </property>
+       </widget>
+      </item>
+     </layout>
     </widget>
    </widget>
    <widget class="QWidget" name="gridLayoutWidget">