Qml/ModPage: add a separator at the bottom of the header

Also removed the one pixed gap cause it made it look odd

src/leek/qml/ModPage.qml

@@ -18,9 +18,10 @@ Kirigami.Page {
         }
 
         Rectangle {
+            id: header
             Layout.fillWidth: true
-            // Leave a one pixel gap for the header separator
-            Layout.topMargin: 1 - root.topPadding
+
+            Layout.topMargin: -root.topPadding
             Layout.leftMargin: -root.leftPadding
             Layout.rightMargin: -root.rightPadding
 
@@ -39,6 +40,11 @@ Kirigami.Page {
                     }
                 }
             }
+
+            Kirigami.Separator {
+                width: header.width
+                anchors.top: header.bottom
+            }
         }
     }
     Controls.Label {