fix: DEL key deletes nodes and deletion is now undoable

- Add 'Delete' to deleteKeyCode so both Backspace and Delete remove nodes
- Call snapshotHistory() in onBeforeDelete (keyboard) and in DetailPanel
  handleDelete (button) so deletions can be undone with Ctrl+Z

frontend/src/components/canvas/CanvasContainer.tsx

@@ -27,7 +27,7 @@ export function CanvasContainer({ onConnect: onConnectProp, onEdgeDoubleClick, o
   const {
     nodes, edges,
     onNodesChange, onEdgesChange,
-    setSelectedNode,
+    setSelectedNode, snapshotHistory,
   } = useCanvasStore()
 
   const activeTheme = useThemeStore((s) => s.activeTheme)
@@ -59,6 +59,8 @@ export function CanvasContainer({ onConnect: onConnectProp, onEdgeDoubleClick, o
         onNodeDragStart={onNodeDragStart}
         nodeTypes={nodeTypes}
         edgeTypes={edgeTypes}
+        deleteKeyCode={['Backspace', 'Delete']}
+        onBeforeDelete={async () => { snapshotHistory(); return true }}
         snapToGrid
         snapGrid={[16, 16]}
         fitView