gui: fix focus stealing from dialogs during session tree rebuild

buildSessionTree() and updateSessionTree() now save/restore
Window.activeFocusItem instead of only promptInput.activeFocus.
This prevents sessionTreeModel.clear() from pulling focus out of
open modal dialogs (NewSessionDialog, NewAgentDialog).

Also add forceActiveFocus() on dialog open so the first field is
immediately focused.

gui/NewAgentDialog.qml

@@ -162,5 +162,8 @@ Dialog {
         }
     }
 
-    onOpened: errorLabel.text = ""
+    onOpened: {
+        errorLabel.text = ""
+        dirField.forceActiveFocus()
+    }
 }

gui/NewSessionDialog.qml

@@ -75,5 +75,6 @@ Dialog {
         errorLabel.text = ""
         nameField.text = ""
         remoteField.text = ""
+        nameField.forceActiveFocus()
     }
 }

gui/main.qml

@@ -102,7 +102,7 @@ ApplicationWindow {
     
     function buildSessionTree() {
         // Full rebuild - used on initial load
-        var hadFocus = promptInput.activeFocus
+        var focusItem = root.activeFocusItem
         sessionTreeModel.clear()
         modelCache = {}
         var sessions = ollie.sessions
@@ -139,8 +139,9 @@ ApplicationWindow {
             
             appendSessionToModel(sess)
         }
-        if (hadFocus)
-            promptInput.forceActiveFocus()
+        // Restore focus to whatever had it (handles dialogs, not just promptInput)
+        if (focusItem && focusItem.visible)
+            focusItem.forceActiveFocus()
     }
     
     // Compute connection color from session data (avoids C++ call)
@@ -197,7 +198,7 @@ ApplicationWindow {
     
     function updateSessionTree() {
         // Incremental update - diff old vs new
-        var hadFocus = promptInput.activeFocus
+        var focusItem = root.activeFocusItem
         var sessions = ollie.sessions
         
         // Build lookup of new sessions
@@ -232,8 +233,9 @@ ApplicationWindow {
             }
         }
         
-        if (hadFocus)
-            promptInput.forceActiveFocus()
+        // Restore focus to whatever had it (handles dialogs, not just promptInput)
+        if (focusItem && focusItem.visible)
+            focusItem.forceActiveFocus()
     }
     
     function findSessionIndex(sessionId) {