Spike: context menu for the Schema View canvas

## Goal

Determine what actions should surface in a context menu on the Schema View canvas. The graph view canvas has a rich context menu with actions for expanding, refreshing, removing, centering, fitting, opening sidebars, and opening style dialogs — all contextual to the selection (single vertex, single edge, multi-selection, or no target). The Schema View currently suppresses the context menu entirely (`onContextMenu={preventContextMenu}` in `SchemaGraph.tsx`).

The Schema View operates on types rather than live graph entities, so many graph-view actions don't apply. But a subset of contextual actions would be valuable — especially around styling, navigation (center/fit, open details), and potentially filtering.

## Questions to resolve via grilling session

- Which graph-view context menu actions carry over to the schema view? (e.g. center, fit, open style dialog, open details sidebar)
- Are there schema-specific actions that don't exist in the graph view? (e.g. filter to this type, copy type name, show connections of this type)
- How does the target model map? Graph view has vertex/edge instances; schema view has vertex types and edge connections.
- Should the no-target (background click) context menu exist? What would it offer? (e.g. fit all, re-layout, toggle labels)
- Does this need coordination with the combined styling sidebar (#1902)?

## Expected Outcome

- A recommendation for the context menu action set per target type (single vertex-type, single edge-connection, multi-selection, no target)
- Tasked-out follow-up issue(s) ready for implementation

## Related Issues

- Related to #1902
- Related to #1885

> [!IMPORTANT]
> Internal only — this issue is maintained by the core team and is not accepting external contributions.