fix: element creation fails with empty layer_id (FOREIGN KEY constraint)

Three fixes:
1. interaction/index.ts: Add layerId to created element in confirmDraw()
2. CanvasArea.tsx: Sync layers to LayerManager via layerManagerRef
3. App.tsx: Fallback to activeLayerId in handleElementCreated if layerId missing

Root cause: LayerManager.activeLayerId was empty because layers were never
synced from React state to the interaction layer LayerManager. New elements
got layer_id="" which failed SQLite FOREIGN KEY constraint.

Also includes Yjs sync race condition fix from previous commit.

frontend/src/components/CanvasArea.tsx

@@ -29,6 +29,7 @@ const CanvasArea: React.FC<CanvasAreaProps> = ({
   const renderEngineRef = useRef<RenderEngine | null>(null);
   const interactionRef = useRef<InteractionEngine | null>(null);
   const spatialIndexRef = useRef<SpatialIndex | null>(null);
+  const layerManagerRef = useRef<LayerManager | null>(null);
 
   useEffect(() => {
     const canvas = canvasRef.current;
@@ -48,6 +49,7 @@ const CanvasArea: React.FC<CanvasAreaProps> = ({
     renderEngineRef.current = renderEngine;
     interactionRef.current = interaction;
     spatialIndexRef.current = spatialIndex;
+    layerManagerRef.current = layerManager;
 
     interaction.setCallbacks({
       onElementCreated,
@@ -68,6 +70,7 @@ const CanvasArea: React.FC<CanvasAreaProps> = ({
       renderEngineRef.current = null;
       interactionRef.current = null;
       spatialIndexRef.current = null;
+      layerManagerRef.current = null;
     };
   // eslint-disable-next-line react-hooks/exhaustive-deps
   }, []);
@@ -133,7 +136,12 @@ const CanvasArea: React.FC<CanvasAreaProps> = ({
   // Sync layers to LayerManager + render
   useEffect(() => {
     const renderEngine = renderEngineRef.current;
+    const layerManager = layerManagerRef.current;
     if (!renderEngine) return;
+    if (layerManager) {
+      layerManager.clear();
+      layers.forEach(l => layerManager.addLayer(l));
+    }
     renderEngine.setLayers(layers);
     renderEngine.render();
   // eslint-disable-next-line react-hooks/exhaustive-deps