feat: make resolveNode recursive for nested subgraphs (depth > 1)

## Summary

The `resolveNode` utility in `src/utils/litegraphUtil.ts` currently iterates only over `graph.subgraphs.values()` at depth 1. This means nodes nested inside subgraphs that are themselves inside other subgraphs (depth > 1) will not be found.

## Background

This limitation was identified in PR #10009 (comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/10009), which expanded the usage of `resolveNode` to fix image copy/paste and display for nodes inside subgraphs. The single-level limitation is pre-existing and was not introduced by that PR.

## Current Behavior

`resolveNode` searches the root graph first, then iterates `graph.subgraphs.values()` — but only one level deep. Nodes inside a subgraph that is itself nested inside another subgraph will not be resolved.

## Expected Behavior

`resolveNode` should recursively search all subgraphs at any depth so that nodes at any level of nesting are found.

## Impact

- Image copy/paste and display for nodes inside nested subgraphs (depth > 1) will silently fail.
- Any other call site using `resolveNode` will also miss deeply-nested nodes.

## Follow-up

This should be addressed when nested subgraphs become more common or when deeper nesting is officially supported.

Requested by @christian-byrne in PR #10009.

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