Fix nodes ui: Make nodes dot background to be the same as the snap to grid size and position

Fix nodes ui:  Make nodes dot background to be the same as the snap to grid size and position
Update to Flow.tsx

Changes the size and offset of the dots background to be the same size as the snap to grid, and also fix the background dot pattern alignment.

Currently, the snapGrid is 25x25, and the default background dot gap is 20x20, these do not align.  This is fixed by making the gap property of the background the same as the snapGrid.

Additionally, there is a bug in the rectFlow background code that incorrectly sets the offset to be the centre of the dot pattern with the default offset of 0.  To work around this issue, setting the background offset property to the snapGrid size will realign the dot pattern correctly. 

I have logged a bug for the rectFlow background issue in its repo. 
xyflow/xyflow#5405

Author: skunkworxdark

invokeai/frontend/web/src/features/nodes/components/flow/Flow.tsx

@@ -263,7 +263,7 @@ export const Flow = memo(() => {
         noWheelClassName={NO_WHEEL_CLASS}
         noPanClassName={NO_PAN_CLASS}
       >
-        <Background />
+        <Background gap={snapGrid} offset={snapGrid} />
       </ReactFlow>
       <HotkeyIsolator />
     </>