fix(gui): load sessions on startup, show '(no sessions)' placeholder

- ensureRootDataLoaded() now calls refreshSessions() so the initial
  session list is populated immediately, not only on events
- Added '(no sessions)' placeholder label in the session tree ListView
  when no sessions exist

gui/main.qml

@@ -549,6 +549,16 @@ ApplicationWindow {
                 model: sessionTreeModel
                 clip: true
 
+                // Show placeholder when empty
+                Label {
+                    anchors.centerIn: parent
+                    text: "(no sessions)"
+                    color: theme.midColor
+                    font.italic: true
+                    font.pixelSize: theme.fontSize > 0 ? theme.fontSize * 0.9 : 12
+                    visible: sessionList.count === 0
+                }
+
                 delegate: ItemDelegate {
                     width: sessionList.width
 

gui/ollie9pclient.cpp

@@ -424,6 +424,7 @@ void Ollie9pClient::ensureRootDataLoaded()
             emit rootModelsLoadedChanged();
         }
     }
+    refreshSessions();
 }
 
 // --- Streaming ---