fix: memoize onBeforeDelete and fix test mock selectors

Extract onBeforeDelete into useCallback to avoid new fn ref each render.
Fix useThemeStore/useCanvasStore mocks to call the selector fn rather
than ignoring it, and reset mockZoom in beforeEach.

frontend/src/components/canvas/CanvasContainer.tsx

@@ -73,6 +73,11 @@ export function CanvasContainer({ onConnect: onConnectProp, onEdgeDoubleClick, o
     onNodeDoubleClick?.(node)
   }, [onNodeDoubleClick])
 
+  const handleBeforeDelete = useCallback(async () => {
+    snapshotHistory()
+    return true
+  }, [snapshotHistory])
+
   return (
     <div className="w-full h-full" style={{ background: theme.colors.canvasBackground }}>
       <ReactFlow
@@ -89,7 +94,7 @@ export function CanvasContainer({ onConnect: onConnectProp, onEdgeDoubleClick, o
         nodeTypes={nodeTypes}
         edgeTypes={edgeTypes}
         deleteKeyCode={['Backspace', 'Delete']}
-        onBeforeDelete={async () => { snapshotHistory(); return true }}
+        onBeforeDelete={handleBeforeDelete}
         selectionOnDrag={lassoMode}
         panOnDrag={lassoMode ? [1, 2] : true}
         panActivationKeyCode="Space"