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

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

(cherry picked from commit ec2cd99405)

eeschema/sch_edit_frame.cpp

@@ -1117,6 +1117,10 @@ 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,
@@ -1147,6 +1151,10 @@ 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 = AllSchematicFilesWildcard()
                             + wxS( "|" ) + KiCadSchematicFileWildcard()

pcbnew/files.cpp

@@ -315,6 +315,10 @@ 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 = 0;
         wxString    fileName = Prj().AbsolutePath( GetBoard()->GetFileName() );
 
@@ -324,6 +328,10 @@ 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 = 1;
         wxString    fileName; // = Prj().AbsolutePath( GetBoard()->GetFileName() );
 
@@ -382,6 +390,10 @@ 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();