feat(editor): snap dragged nodes to the 20px grid

Match Vue Flow's snap grid to the background gap so nodes line up
on the dots they already render against.

src/components/Editor.vue

@@ -385,6 +385,8 @@ onBeforeUnmount(() => {
         :connection-radius="30"
         :min-zoom="0.2"
         :max-zoom="4"
+        :snap-to-grid="true"
+        :snap-grid="[20, 20]"
         class="size-full"
       >
         <Background :gap="20" pattern-color="#d1d5db" />