feat: safe workspace tree deletion with reference cleanup and confirmation

- Add deleteRootStruct() that clears orphaned refId references before removal
- Show confirmation dialog listing all fields that reference the deleted type
- Auto-switch view to next root struct when the viewed one is deleted
- Entire operation is a single undo macro (Ctrl+Z restores everything)

src/controller.h

@@ -99,6 +99,7 @@ public:
     void showContextMenu(RcxEditor* editor, int line, int nodeIdx, int subLine, const QPoint& globalPos);
     void batchRemoveNodes(const QVector<int>& nodeIndices);
     void batchChangeKind(const QVector<int>& nodeIndices, NodeKind newKind);
+    void deleteRootStruct(uint64_t structId);
 
     void applyCommand(const Command& cmd, bool isUndo);
     void refresh();