gui: fix text selection in prompt box

Replace MouseArea overlay with TapHandler inside TextArea.
The MouseArea sat on top and intercepted left-button press events,
blocking click-and-drag text selection. TapHandler cooperates with
the TextArea's built-in selection handling.

kde/gui/ChatPane.qml

@@ -253,15 +253,12 @@ Item {
                                     event.accepted = true
                                 }
                             }
-                        }
-                    }
 
-                    // Context menu overlay - only catches right-click, on top of ScrollView
+                            TapHandler {
-                    MouseArea {
-                        anchors.fill: parent
                                 acceptedButtons: Qt.RightButton
-                        cursorShape: Qt.IBeamCursor
+                                onTapped: promptContextMenu.popup()
-                        onClicked: promptContextMenu.popup()
+                            }
+                        }
                     }
 
                     Menu {