fix: stale closure bug preventing canvas tools from creating elements

- CanvasArea: add useEffect to refresh InteractionEngine callbacks when props change
- App.tsx: handleElementCreated/Deleted/Modified now use functional setElements updates
- Root cause: CanvasArea init useEffect had empty deps, capturing stale callbacks at mount
- handleElementCreated used [...elements, el] with stale elements=[] closure
- Each new element replaced all previous ones instead of appending
- All 343 tests pass, tsc clean

frontend/src/App.tsx

@@ -293,11 +293,13 @@ const CADEditor: React.FC<CADEditorProps> = ({ projectId, token, onNavigateBack
   }, [collab.elements, collab.layers, collab.blocks, collab.status]);
 
   const handleElementCreated = useCallback((el: CADElement) => {
-    const newElements = [...elements, el];
-    historyManagerRef.current.pushSnapshot({
-      elements: newElements, layers, blocks, groups, bgConfig,
-    }, 'Element erstellt');
-    setElements(newElements);
+    setElements((prev) => {
+      const newElements = [...prev, el];
+      historyManagerRef.current.pushSnapshot({
+        elements: newElements, layers, blocks, groups, bgConfig,
+      }, 'Element erstellt');
+      return newElements;
+    });
     syncHistory();
     // Push to Yjs CRDT for real-time sync
     collab.setElement(el);
@@ -311,14 +313,16 @@ const CADEditor: React.FC<CADEditorProps> = ({ projectId, token, onNavigateBack
         setSavedStatus('Fehler beim Speichern');
       });
     }
-  }, [elements, layers, blocks, groups, bgConfig, syncHistory, drawingId, token, collab]);
+  }, [layers, blocks, groups, bgConfig, syncHistory, drawingId, token, collab]);
 
   const handleElementsDeleted = useCallback((ids: string[]) => {
-    const newElements = elements.filter((e) => !ids.includes(e.id));
-    historyManagerRef.current.pushSnapshot({
-      elements: newElements, layers, blocks, groups, bgConfig,
-    }, `${ids.length} Element(e) gelöscht`);
-    setElements(newElements);
+    setElements((prev) => {
+      const newElements = prev.filter((e) => !ids.includes(e.id));
+      historyManagerRef.current.pushSnapshot({
+        elements: newElements, layers, blocks, groups, bgConfig,
+      }, `${ids.length} Element(e) gelöscht`);
+      return newElements;
+    });
     syncHistory();
     // Push deletions to Yjs CRDT for real-time sync
     ids.forEach(id => collab.deleteElement(id));
@@ -335,14 +339,16 @@ const CADEditor: React.FC<CADEditorProps> = ({ projectId, token, onNavigateBack
   }, [elements, layers, blocks, groups, bgConfig, syncHistory, token, collab]);
 
   const handleElementsModified = useCallback((modified: CADElement[]) => {
-    const newElements = elements.map((e) => {
-      const found = modified.find((m) => m.id === e.id);
-      return found ?? e;
+    setElements((prev) => {
+      const newElements = prev.map((e) => {
+        const found = modified.find((m) => m.id === e.id);
+        return found ?? e;
+      });
+      historyManagerRef.current.pushSnapshot({
+        elements: newElements, layers, blocks, groups, bgConfig,
+      }, `${modified.length} Element(e) geändert`);
+      return newElements;
     });
-    historyManagerRef.current.pushSnapshot({
-      elements: newElements, layers, blocks, groups, bgConfig,
-    }, `${modified.length} Element(e) geändert`);
-    setElements(newElements);
     syncHistory();
     // Push modifications to Yjs CRDT for real-time sync
     modified.forEach(el => collab.setElement(el));
@@ -356,7 +362,7 @@ const CADEditor: React.FC<CADEditorProps> = ({ projectId, token, onNavigateBack
         setSavedStatus('Fehler beim Speichern');
       });
     }
-  }, [elements, layers, blocks, groups, bgConfig, syncHistory, token, collab]);
+  }, [layers, blocks, groups, bgConfig, syncHistory, token, collab]);
 
   const lastCursorUpdateRef = useRef(0);
   const handleCursorMoved = useCallback((x: number, y: number) => {

frontend/src/components/CanvasArea.tsx

@@ -72,6 +72,23 @@ const CanvasArea: React.FC<CanvasAreaProps> = ({
   // eslint-disable-next-line react-hooks/exhaustive-deps
   }, []);
 
+  // Update callbacks when they change (avoids stale closures)
+  useEffect(() => {
+    const interaction = interactionRef.current;
+    if (!interaction) return;
+    interaction.setCallbacks({
+      onElementCreated,
+      onElementsDeleted,
+      onElementsModified,
+      onCursorMoved,
+      onToolStateChanged,
+      onTextEdit,
+      onCommandTrigger,
+      onSelectionChange,
+    });
+  // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [onElementCreated, onElementsDeleted, onElementsModified, onCursorMoved, onToolStateChanged, onTextEdit, onCommandTrigger, onSelectionChange]);
+
   // Resize canvas to fill container
   useEffect(() => {
     const canvas = canvasRef.current;