Check for updates on a ComfyBuilder install shows raw API error when its distribution is not available in the active workspace

## Bug

Clicking **Check for updates** on a ComfyBuilder-sourced install whose distribution is not available under the currently active workspace (e.g. the install came from a workspace that is not the one signed in / selected, or the distribution was deactivated) shows the raw backend error verbatim in the dialog:

```
/v1/distributions/d33b540e-a04e-4ac5-9262-12a2ce1a68b7/versions not found: NOT_FOUND
```

This is user-hostile messaging: a distribution UUID and an HTTP error code tell the user nothing about what happened or what to do.

## Expected

A plain-language message explaining that the install's distribution is not available in the current workspace, and what to do about it (e.g. switch to the workspace that provides this distribution, or that the distribution may no longer be published). The update check should distinguish "distribution not found / not accessible in this workspace" (404) from genuine transport or server failures.

## Where

- `src/main/comfybuilder/client.ts` (~L164-203): builds the raw ` not found: NOT_FOUND` message from the HTTP response; the typed error kind/status is available here.
- `src/main/sources/comfybuilder/index.ts` (~L559-587) and `src/main/sources/comfybuilder/detailSections.ts` (~L145-167): the source's check-for-updates path returns the client error message unchanged.
- `src/renderer/src/composables/useComfyUISettings.ts` (~L580-626): displays the returned string verbatim in the "Check for updates" alert dialog.

## Notes

- Report only for now; fix deferred.
- Related: the same raw-passthrough pattern likely affects other builder actions that hit workspace-scoped endpoints while the wrong workspace is active (update install, version listing in Manage). Worth auditing when fixing.

Start in src/main/comfybuilder/client.ts around lines 164-203 to trace the typed HTTP error and 404 handling. Then follow the update-check path in src/main/sources/comfybuilder/index.ts and detailSections.ts into useComfyUISettings.ts. Done means the alert explains workspace unavailability or an unpublished distribution in plain language, while genuine transport and server failures remain distinct.

Written by the indexing model from the issue text.