jobsets: fix PDF background output

Jobset PDF backgrounds were drawn from (0,0) and ignored the plotter’s
offset, so the fill didn’t line up with the plotted content. Use the
plotter’s page size plus its plot offset when drawing the background
rectangle. This makes jobset output match the manual UI plot without
changing other plot behavior.

Fixes: https://gitlab.com/kicad/code/kicad/-/issues/22793

pcbnew/plot_board_layers.cpp

@@ -1214,18 +1214,20 @@ static void FillNegativeKnockout( PLOTTER *aPlotter, const BOX2I &aBbbox )
 
 static void plotPdfBackground( BOARD* aBoard, const PCB_PLOT_PARAMS* aPlotOpts, PLOTTER* aPlotter )
 {
+    const PAGE_INFO& pageInfo = aPlotter->PageSettings();
+    const VECTOR2I   plotOffset = aPlotter->GetPlotOffsetUserUnits();
+    const VECTOR2I   pageSizeIU( pageInfo.GetWidthIU( pcbIUScale.IU_PER_MILS ),
+                                 pageInfo.GetHeightIU( pcbIUScale.IU_PER_MILS ) );
+
     if( aPlotter->GetColorMode()
         && aPlotOpts->GetPDFBackgroundColor() != COLOR4D::UNSPECIFIED )
     {
         aPlotter->SetColor( aPlotOpts->GetPDFBackgroundColor() );
 
-        // Use page size selected in pcb to know the schematic bg area
-        const PAGE_INFO& actualPage = aBoard->GetPageSettings();
+        // Use plotter page size and offset so background matches the plotted output.
+        VECTOR2I end = plotOffset + pageSizeIU;
 
-        VECTOR2I end( actualPage.GetWidthIU( pcbIUScale.IU_PER_MILS ),
-                      actualPage.GetHeightIU( pcbIUScale.IU_PER_MILS ) );
-
-        aPlotter->Rect( VECTOR2I( 0, 0 ), end, FILL_T::FILLED_SHAPE, 1.0 );
+        aPlotter->Rect( plotOffset, end, FILL_T::FILLED_SHAPE, 1.0 );
     }
 }