feat: surface user-visible retry/error when create-workspace dialog fails to load from URL

## Background

In `src/platform/workspace/composables/useCreateWorkspaceUrlLoader.ts`, the URL trigger (`?create_workspace=1`) and preserved query are intentionally cleaned up **before** `dialogService.showCreateWorkspaceDialog()` is awaited. This ordering prevents an infinite reload loop if the lazy-loaded dialog component consistently fails to load.

However, when `showCreateWorkspaceDialog()` rejects, the user currently only receives a `console.error` with no actionable feedback or retry path.

## Task

Improve the error UX in `useCreateWorkspaceUrlLoader.loadCreateWorkspaceFromUrl()` when the dialog fails to open:
- Surface a user-visible error (e.g., a toast notification) so the user knows the action failed.
- Optionally provide a retry action (e.g., a toast button that re-triggers the dialog) without restoring the URL (to keep the no-reload-loop guarantee).
- Keep the cleanup-first ordering to preserve the reload-loop guard.

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9901
- Comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9901#discussion_r2944311962
- Requested by: @christian-byrne

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-10214-feat-surface-user-visible-retry-error-when-create-workspace-dialog-fails-to-load-fro-3266d73d3650810c9369fa5ca6abe9ca) by [Unito](https://www.unito.io)