Push node background-color to Cytoscape ele.data() with styled type atoms

## Description

The current system rebuilds a per-type Cytoscape stylesheet (up to 70k selectors) whenever any style preference changes. This is the primary performance bottleneck for large schemas.

This slice validates a new architecture end-to-end with a single property (`background-color`):

1. Add `styledVertexTypeAtom(type)` / `styledEdgeTypeAtom(type)` — new per-type atoms that compose `graphSchemaAtom` + preference atoms, returning `{ schema, preferences }`
2. When creating Cytoscape nodes, set `ge_color` in `ele.data()` from the new styled atoms
3. Add a `data(ge_color)` mapper to the Cytoscape stylesheet for `background-color`, alongside the existing per-type selector
4. Add a `useSyncStylesToGraph` hook that batch-updates `ele.data()` when a preference changes for a given type

The old per-type selector system continues to work for all other properties. This slice proves the architecture works before committing to a full migration.

## Preferred Solution

- New atoms: `styledVertexTypeAtom(type)` and `styledEdgeTypeAtom(type)` that return `{ schema: VertexSchema, preferences: VertexPreferences }`
- Use Cytoscape built-in `data()` string mappers (not function mappers) for style resolution
- Use `cy.batch()` to coalesce `ele.data()` updates when preferences change
- Only update elements of the changed type — O(elements of that type), not O(all elements)

## Related Issues

- Part of #1677
- Supersedes #1678

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