refactor: Rename `zeroUuid` to `UUID_ZEROES` as a proper constant

## Summary

The `zeroUuid` export in `src/lib/litegraph/src/utils/uuid.ts` should be renamed to `UUID_ZEROES` to follow proper SCREAMING_SNAKE_CASE naming conventions for module-level constants.

## Motivation

Using `UUID_ZEROES` makes it immediately clear at the call site that this is a constant sentinel value, consistent with naming conventions for well-known fixed values.

## Affected Files

The rename would touch the following files:

- `src/lib/litegraph/src/utils/uuid.ts` — definition
- `src/lib/litegraph/src/LGraph.ts` — 4 usages (default field value, `clear()`, `_configureBase()`)
- `src/lib/litegraph/src/LGraph.test.ts` — 5 usages in the new `Zero UUID handling in configure` describe block
- `src/lib/litegraph/src/subgraph/EmptySubgraphInput.ts` — 1 usage
- `src/lib/litegraph/src/subgraph/EmptySubgraphOutput.ts` — 1 usage

A backward-compatible re-export (`export { UUID_ZEROES as zeroUuid }`) could be kept temporarily if needed for any external consumers.

## Related

Raised in PR #10825 comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/10825#discussion_r3030120796

Requested by @DrJKL.

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-10830-refactor-Rename-zeroUuid-to-UUID_ZEROES-as-a-proper-constant-3366d73d36508189a08ded5b77597bfc) by [Unito](https://www.unito.io)