fix(gui): prevent session list from stealing keyboard focus

- ListView and its placeholder label now have activeFocusOnTab: false
  and focus: false so clicking the empty sidebar doesn't steal focus
  from the prompt text area

gui/main.qml

@@ -550,6 +550,8 @@ ApplicationWindow {
                 Layout.fillHeight: true
                 model: sessionTreeModel
                 clip: true
+                focus: false
+                activeFocusOnTab: false
 
                 // Show placeholder when empty
                 Label {
@@ -559,6 +561,7 @@ ApplicationWindow {
                     font.italic: true
                     font.pixelSize: theme.fontSize > 0 ? theme.fontSize * 0.9 : 12
                     visible: sessionList.count === 0
+                    activeFocusOnTab: false
                 }
 
                 delegate: ItemDelegate {