Guard actions that change project against running in non-standalone mode

Fixes https://gitlab.com/kicad/code/kicad/-/issues/16942

eeschema/sch_edit_frame.cpp

@@ -1260,6 +1260,10 @@ void SCH_EDIT_FRAME::OnClearFileHistory( wxCommandEvent& aEvent )
 
 void SCH_EDIT_FRAME::NewProject()
 {
+    // Only standalone mode can directly load a new document
+    if( !Kiface().IsSingle() )
+        return;
+
     wxString pro_dir = m_mruPath;
 
     wxFileDialog dlg( this, _( "New Schematic" ), pro_dir, wxEmptyString,
@@ -1290,6 +1294,10 @@ void SCH_EDIT_FRAME::NewProject()
 
 void SCH_EDIT_FRAME::LoadProject()
 {
+    // Only standalone mode can directly load a new document
+    if( !Kiface().IsSingle() )
+        return;
+
     wxString pro_dir = m_mruPath;
     wxString wildcards = FILEEXT::AllSchematicFilesWildcard()
                             + wxS( "|" ) + FILEEXT::KiCadSchematicFileWildcard()

pcbnew/files.cpp

@@ -265,6 +265,10 @@ bool PCB_EDIT_FRAME::Files_io_from_id( int id )
     {
     case ID_LOAD_FILE:
     {
+        // Only standalone mode can directly load a new document
+        if( !Kiface().IsSingle() )
+            return false;
+
         int      open_ctl = KICTL_KICAD_ONLY;
         wxString fileName = Prj().AbsolutePath( GetBoard()->GetFileName() );
 
@@ -274,6 +278,10 @@ bool PCB_EDIT_FRAME::Files_io_from_id( int id )
 
     case ID_IMPORT_NON_KICAD_BOARD:
     {
+        // Only standalone mode can directly load a new document
+        if( !Kiface().IsSingle() )
+            return false;
+
         int      open_ctl = KICTL_NONKICAD_ONLY;
         wxString fileName; // = Prj().AbsolutePath( GetBoard()->GetFileName() );
 
@@ -332,6 +340,10 @@ bool PCB_EDIT_FRAME::Files_io_from_id( int id )
 
     case ID_NEW_BOARD:
     {
+        // Only standalone mode can directly load a new document
+        if( !Kiface().IsSingle() )
+            return false;
+
         if( IsContentModified() )
         {
             wxFileName fileName = GetBoard()->GetFileName();