fix: drawing persistence bug — sync activeLayerId on project load + create default layer when none exist

frontend/src/App.tsx

@@ -279,12 +279,30 @@ const CADEditor: React.FC<CADEditorProps> = ({ projectId, token, onNavigateBack
         if (data.elements.length > 0) setElements(data.elements);
         if (data.layers.length > 0) setLayers(data.layers);
         if (data.blocks.length > 0) setBlocks(data.blocks);
-        // Save initial layers to backend if backend has none
+        // Ensure activeLayerId matches a real layer
-        if (data.layers.length === 0 && data.drawing) {
+        if (data.layers.length > 0) {
-          for (const layer of initialLayers) {
+          const hasActiveLayer = data.layers.some(l => l.id === 'layer-0');
-            createLayerTyped(token, data.drawing.id, layer).catch(() => {});
+          if (!hasActiveLayer) {
+            setActiveLayerId(data.layers[0].id);
           }
         }
+        // Create default layer if backend has none
+        if (data.layers.length === 0 && data.drawing) {
+          const defaultLayer: CADLayer = {
+            id: 'layer-0',
+            name: 'Layer 0',
+            visible: true,
+            locked: false,
+            color: '#ffffff',
+            lineType: 'solid',
+            transparency: 0,
+            sortOrder: 0,
+            parentId: null,
+          };
+          createLayerTyped(token, data.drawing.id, defaultLayer).catch(() => {});
+          setLayers([defaultLayer]);
+          setActiveLayerId('layer-0');
+        }
         setSavedStatus('gespeichert');
         setDataLoaded(true);
         // Push initial data to Yjs CRDT after load
@@ -356,6 +374,11 @@ const CADEditor: React.FC<CADEditorProps> = ({ projectId, token, onNavigateBack
       if (mergedLayers.length !== layers.length || mergedLayers.some((ml, i) => ml !== layers[i])) {
         setLayers(mergedLayers);
       }
+      // Ensure activeLayerId matches a real layer after collab merge
+      const hasActiveInMerged = mergedLayers.some(l => l.id === activeLayerId);
+      if (!hasActiveInMerged && mergedLayers.length > 0) {
+        setActiveLayerId(mergedLayers[0].id);
+      }
     }
 
     // Merge blocks: local-first, update changed from remote, append remote-only