feat(ui): snap cursor pos when drawing rects

- Rects snap to stage edge when within a threshold (10 screen pixels)
- When mouse leaves stage, set last mousedown pos to null, preventing nonfunctional rect outlines

Partially addresses #6306.

There's a technical challenge to fully address the issue - mouse event are not fired when the mouse is outside the stage. While we could draw the rect even if the mouse leaves, we cannot update the rect's dimensions on mouse move, or complete the drawing on mouse up.

To fully address the issue, we'd need to a way to forward window events back to the stage, or at least handle window events. We can explore this later.

invokeai/frontend/web/src/features/controlLayers/util/renderers.ts

@@ -1,6 +1,6 @@
 import { getStore } from 'app/store/nanostores/store';
 import { rgbaColorToString, rgbColorToString } from 'features/canvas/util/colorToString';
-import { getScaledFlooredCursorPosition } from 'features/controlLayers/hooks/mouseEventHooks';
+import { getScaledFlooredCursorPosition, snapPosToStage } from 'features/controlLayers/hooks/mouseEventHooks';
 import {
   $tool,
   BACKGROUND_LAYER_ID,
@@ -211,12 +211,13 @@ const renderToolPreview = (
   }
 
   if (cursorPos && lastMouseDownPos && tool === 'rect') {
+    const snappedPos = snapPosToStage(cursorPos, stage);
     const rectPreview = toolPreviewLayer.findOne<Konva.Rect>(`#${TOOL_PREVIEW_RECT_ID}`);
     rectPreview?.setAttrs({
-      x: Math.min(cursorPos.x, lastMouseDownPos.x),
-      y: Math.min(cursorPos.y, lastMouseDownPos.y),
-      width: Math.abs(cursorPos.x - lastMouseDownPos.x),
-      height: Math.abs(cursorPos.y - lastMouseDownPos.y),
+      x: Math.min(snappedPos.x, lastMouseDownPos.x),
+      y: Math.min(snappedPos.y, lastMouseDownPos.y),
+      width: Math.abs(snappedPos.x - lastMouseDownPos.x),
+      height: Math.abs(snappedPos.y - lastMouseDownPos.y),
     });
     rectPreview?.visible(true);
   } else {