Bug: Editing SQL-Generated Diagram Creates a New Diagram Instead of Updating the Existing One

**Summary**

The recently introduced workflow from #6 (Export Schema as SQL Query) and #7 (Add Per-Table SQL Copy Option) enables users to export schema definitions and reuse them when creating new diagrams.

However, when returning to an existing SQL-generated diagram and using the Edit Diagram functionality to add additional table definitions, **SchemaVis** creates an entirely new diagram instead of updating the original diagram.

This prevents diagrams from functioning as an iterative schema workspace and breaks the expected edit-and-expand workflow.

This issue affects the workflow enabled by:

#6 Export Schema as SQL Query
#7 Add Per-Table SQL Copy Option

**The intended workflow is:**

Visualize → Export/Copy SQL → Create Diagram → Edit Diagram → Add More Tables → Continue Expanding Same Diagram

**Currently, the workflow becomes:**

Visualize → Export/Copy SQL → Create Diagram → Edit Diagram → New Diagram Created

which prevents incremental schema development and introduces unnecessary duplication.