Legend should only show vertex types visible on the canvas

## Description

The graph viewer legend currently shows all vertex types from the schema, not just the types that are visible on the canvas. For schemas with many types (70k+), this makes the legend unusable. Even for moderate schemas, showing types that have no nodes on the canvas adds noise.

## Preferred Solution

Filter the legend entries to only include vertex types that have at least one node currently on the canvas. The legend should update reactively as nodes are added or removed.

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