refactor: abstract node-visibility AABB check into a standalone utility

## Summary

The inline AABB (axis-aligned bounding box) visibility check added in #10810 (`src/stores/subgraphNavigationStore.ts`) is duplicated in at least one other location (`src/scripts/app.ts` lines 1272–1281). It should be extracted into a reusable, well-named standalone utility function.

## Context

- PR: #10810 (comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/10810#discussion_r3046136063)
- Requested by: @DrJKL

## Proposed utility signature (standalone function, not a method on LGraphCanvas)

Per the project ADR (ADR 0008), new behavior must **not** be added as methods to `LGraphNode`, `LGraphCanvas`, `LGraph`, or `Subgraph`. The utility should live in an appropriate location such as `src/utils/graphTraversalUtil.ts` or a new `src/utils/viewportUtil.ts`.

```ts
/**
* Returns true if at least one node's bounding box intersects the canvas
* visible area (i.e. some content is currently on screen).
*/
function hasVisibleNodes(canvas: LGraphCanvas): boolean
```

The implementation would encapsulate:
1. `canvas.ds.computeVisibleArea(canvas.viewport)`
2. Reading `canvas.visible_area`
3. The AABB intersection check over `canvas.graph.nodes`

## Acceptance criteria

- [ ] Extract the logic into a standalone exported utility (not a class method on any LiteGraph entity).
- [ ] Replace the inline check in `subgraphNavigationStore.ts` with the utility call.
- [ ] Replace or consolidate the equivalent check in `app.ts` (lines ~1272–1281) if applicable.
- [ ] Add unit tests for the utility.

/cc @artokun

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