fix(refboard): drag-drop opening in new tab — use ref for drop target

The drag/drop setup was searching for a <canvas> element to find its
parent, which was fragile and could fail (returning null). When it
failed, document-level dragover/drop prevention was never registered,
so the browser's default behavior (open file in new tab) kicked in.

Fix: pass a direct ref to the outer canvas container div from Editor
to useCanvasSetup. No more DOM querying for the drop target.

frontend/src/hooks/useCanvasSetup.ts

@@ -40,6 +40,7 @@ interface CanvasSetupDeps {
   undoRef: React.MutableRefObject<UndoManager | null>;
   syncRef: React.MutableRefObject<SyncHandle | null>;
   inboxZoneRef: React.MutableRefObject<InboxZone | null>;
+  canvasContainerRef: React.RefObject<HTMLDivElement | null>;
   onCanvasChange: (changedIds?: string[]) => void;
   showToast: (msg: string) => void;
   setOnlineUsers: React.Dispatch<React.SetStateAction<OnlineUser[]>>;
@@ -53,7 +54,7 @@ interface CanvasSetupDeps {
 export function useCanvasSetup(deps: CanvasSetupDeps) {
   const {
     boardData, resolvedBoardId, user, isPublicView,
-    canvasRef, selectionRef, undoRef, syncRef, inboxZoneRef,
+    canvasRef, selectionRef, undoRef, syncRef, inboxZoneRef, canvasContainerRef,
     onCanvasChange, showToast, setOnlineUsers, setSelectedLayerIds,
   } = deps;
 
@@ -252,21 +253,11 @@ export function useCanvasSetup(deps: CanvasSetupDeps) {
 
       // Setup drag/drop and paste
       if (!isPublicView || user) {
-        const canvasElements = document.querySelectorAll('canvas');
-        let domContainer: HTMLElement | null = null;
-        for (const c of canvasElements) {
-          if (c.parentElement && c.width > 100) {
-            domContainer = c.parentElement;
-            break;
+        const dropTarget = canvasContainerRef.current;
+        if (dropTarget) {
+          dropCleanupRef.current = setupDragDrop(dropTarget, viewport, scene, resolvedBoardId, onCanvasChange);
         }
-        }
-
-        if (domContainer) {
-          dropCleanupRef.current = setupDragDrop(domContainer, viewport, scene, resolvedBoardId, onCanvasChange);
         pasteCleanupRef.current = setupPaste(viewport, scene, resolvedBoardId, onCanvasChange);
-        } else {
-          pasteCleanupRef.current = setupPaste(viewport, scene, resolvedBoardId, onCanvasChange);
-        }
       }
     }, 200);
 

frontend/src/pages/Editor.tsx

@@ -59,6 +59,7 @@ export default function Editor({ isPublicView }: EditorProps) {
   const toolCleanupRef = useRef<(() => void) | null>(null);
   const inboxZoneRef = useRef<InboxZone | null>(null);
   const clipboardRef = useRef<SceneItem[]>([]);
+  const canvasContainerRef = useRef<HTMLDivElement>(null);
 
   // UI state
   const [activeTool, setActiveTool] = useState<ToolType>(ToolType.SELECT);
@@ -136,7 +137,7 @@ export default function Editor({ isPublicView }: EditorProps) {
   // Canvas setup (selection, undo, sync, socket, drag/drop, paste, inbox)
   useCanvasSetup({
     boardData, resolvedBoardId, user, isPublicView,
-    canvasRef, selectionRef, undoRef, syncRef, inboxZoneRef,
+    canvasRef, selectionRef, undoRef, syncRef, inboxZoneRef, canvasContainerRef,
     onCanvasChange, showToast, setOnlineUsers, setSelectedLayerIds,
   });
 
@@ -473,7 +474,7 @@ export default function Editor({ isPublicView }: EditorProps) {
       />}
 
       {/* Canvas */}
-      <div style={{ flex: 1, position: 'relative', overflow: 'hidden', background: '#1e1e1e' }} onContextMenu={handleContextMenu}>
+      <div ref={canvasContainerRef} style={{ flex: 1, position: 'relative', overflow: 'hidden', background: '#1e1e1e' }} onContextMenu={handleContextMenu}>
         {/* Dot grid — behind transparent canvas */}
         {showGrid && (() => {
           const scale = canvasTransform[0] || 1;