GUI: use a spinbox for ambient temperature instead of a text entry

Replaces lineEdit_ambient (QLineEdit) with doubleSpinBox_ambient
(QDoubleSpinBox, 0-50 degC, 0.5 deg steps) in brewpi.ui, regenerated
main_window.py via pyuic5 (same generator version, verified minimal diff).

brewpi_gui.py wires valueChanged instead of returnPressed - no more
manual float()/ValueError parsing of typed text, the signal already
hands over a valid float - with blockSignals guards around server-driven
updates (mirroring the existing doubleSpinBox_heatrate_soll pattern) so
applying a server echo doesn't immediately re-send the same value back.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DkkuG48uHFCGKe6dPSERFk

client/brewpi.ui

@@ -3714,7 +3714,7 @@
      <set>Qt::AlignCenter</set>
     </property>
    </widget>
-   <widget class="QLineEdit" name="lineEdit_ambient">
+   <widget class="QDoubleSpinBox" name="doubleSpinBox_ambient">
     <property name="geometry">
      <rect>
       <x>620</x>
@@ -3724,7 +3724,19 @@
      </rect>
     </property>
     <property name="toolTip">
-     <string>Set the simulated/assumed ambient temperature - press Enter to apply</string>
+     <string>Set the simulated/assumed ambient temperature</string>
+    </property>
+    <property name="decimals">
+     <number>1</number>
+    </property>
+    <property name="minimum">
+     <double>0.000000000000000</double>
+    </property>
+    <property name="maximum">
+     <double>50.000000000000000</double>
+    </property>
+    <property name="singleStep">
+     <double>0.500000000000000</double>
     </property>
    </widget>
    <widget class="QPushButton" name="btn_pot_reset">