gui: init previewProcessBar with 0 on start

gui/mainwindow.ui

@@ -356,7 +356,7 @@
     <item>
      <widget class="QProgressBar" name="previewProcessBar">
       <property name="value">
-       <number>24</number>
+       <number>0</number>
       </property>
      </widget>
     </item>