GGUF detection: flux gguf models in shared folder not detected after upgrade

Source: Typeform feedback from a 1-year+ Windows user (bjnle33@gmail.com) on v1.0.9:

> "Can't detect flux gguf when its already in folder"

## Triage

This may be one of three things — needs more info to distinguish:

1. **GGUF loader nodes missing** — the user might be using a workflow with `UNetLoaderGGUF` / `DualCLIPLoaderGGUF` from city96's [ComfyUI-GGUF](https://github.com/city96/ComfyUI-GGUF) pack, which may not have migrated cleanly. Desktop doesn't auto-install custom node packs after upgrade.

2. **modelsDirs gap** — #930 carried over all legacy model directories, but if the user kept their gguf files in a non-standard subdir not enumerated by ComfyUI's default model paths, the loader nodes won't find them. We should verify the migration enumerates *every* subdir under the legacy ComfyUI/models/, not just the well-known ones.

3. **extra_models_config.yaml not pointing at gguf-specific paths** — if the user had a custom `extra_models_config.yaml` with `unet:` or `clip:` paths for gguf, the migration may not be re-emitting those entries in the new install's config.

## Suggested actions

- [ ] Reach out to bjnle33 for their `comfyui.log` + `extra_models_config.yaml` + the specific workflow JSON
- [ ] Audit #930 / migration code for whether non-standard subdirs under legacy `ComfyUI/models/` are carried over
- [ ] Confirm with the GGUF custom node pack whether it has any first-load discovery quirks that would be sensitive to the new Desktop's install layout

Day-3 launch context: medium-priority — single user report so far but GGUF is popular enough that if this is real it'll surface more reports this week.