fix(refboard): full sync audit — batch multi-drag, persist resize, sync lock

- Batch broadcastTransform: sends all selected items in one emit instead
  of throttle-dropping all but the first (multi-select drag now syncs all)
- TransformBox onDragEnd: persist+sync final state after resize/rotate
  (was only broadcasting ephemeral transforms, never the final position)
- Layer lock toggle: add missing onCanvasChange() call for sync+save
- Receiver handles both batched and legacy single-item transform formats

frontend/src/canvas/SelectionManager.ts

@@ -40,6 +40,7 @@ export class SelectionManager {
   private _bandGfx: Graphics;
   private _onSelectionChange: ((ids: string[]) => void) | null = null;
   private _onItemTransform: ((item: SceneItem) => void) | null = null;
+  private _onItemsTransform: ((items: SceneItem[]) => void) | null = null;
 
   // Pointer state
   private _pointerDown = false;
@@ -116,6 +117,11 @@ export class SelectionManager {
     this._onItemTransform = fn;
   }
 
+  /** Called during drag with ALL moved items for batched sync broadcast. */
+  set onItemsTransform(fn: (items: SceneItem[]) => void) {
+    this._onItemsTransform = fn;
+  }
+
   /** Called on double-click of a text item (for inline editing). */
   set onDoubleClickText(fn: (item: SceneItem) => void) {
     this._onDoubleClickText = fn;
@@ -260,13 +266,17 @@ export class SelectionManager {
         ddy += snap.dy;
         this._snapGuides.drawGuides(snap.guides, this._viewport);
 
-        // Move all selected items by corrected delta and broadcast transforms
+        // Move all selected items by corrected delta and broadcast all together
         for (const item of selected) {
           item.displayObject.x += ddx;
           item.displayObject.y += ddy;
           item.data.x = item.displayObject.x;
           item.data.y = item.displayObject.y;
-          this._onItemTransform?.(item);
+        }
+        if (this._onItemsTransform) {
+          this._onItemsTransform(selected);
+        } else if (this._onItemTransform) {
+          for (const item of selected) this._onItemTransform(item);
         }
         this.transformBox.update(selected);
       }