Grid move: fallback to prevent flicker (#6210)

**Problem:**

When moving a grid element, if the mouse goes over a non-grid-target
area the element will temporarily jump back to its original position,
creating effectively a flicker effect during the interaction.

**Fix:**

If the cell under the mouse is not found, instead of bailing try to
fallback to the last known valid cell target instead.

| Before | After |
|----------|------------|
| ![Kapture 2024-08-08 at 15 34
38](https://github.com/user-attachments/assets/2114618c-2782-4b87-9aba-ad043d3ddda3)
| ![Kapture 2024-08-08 at 15 34
04](https://github.com/user-attachments/assets/cd21df54-f311-46f1-8ee5-14fe6215fa46)
|


Fixes #6209

editor/src/components/canvas/canvas-strategies/strategies/grid-helpers.ts

@@ -134,13 +134,16 @@ export function runGridRearrangeMove(
     canvasOffset,
   )
 
-  const newTargetCell = getTargetCell(
+  const targetCellUnderMouse = getTargetCell(
     customState.targetCell,
     canvasScale,
     duplicating,
     mouseWindowPoint,
   )?.gridCellCoordinates
 
+  // if there's no cell target under the mouse, try using the last known cell
+  const newTargetCell = targetCellUnderMouse ?? customState.targetCell ?? null
+
   if (newTargetCell == null) {
     return {
       commands: [],