Implement atomic workflow overwrite via persistence-level flag

## Problem

The current workflow save/overwrite flow deletes the existing workflow before the replacement is durably saved (e.g., in `saveWorkflowAs` and `saveWorkflow`). If a subsequent rename, open, or save step fails, the original workflow is already gone, risking data loss.

Affected locations:
- `src/platform/workflow/core/services/workflowService.ts` Line 131 (in `saveWorkflowAs`)
- `src/platform/workflow/core/services/workflowService.ts` Line 173 (in `saveWorkflow`)

## Proposed Solution

Add an `overwrite` flag (or similar) to the workflow persistence layer (`saveWorkflow` / `persistWorkflow` API) that allows atomic replacement:
- Write the new workflow content first
- Only delete/replace the old workflow after successful commit
- Ensure any write failures abort the delete and propagate errors up

This would replace the current pattern of explicit `deleteWorkflow` calls before saving.

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9338
- Comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9338#discussion_r2878008918
- Requested by: @pythongosssss

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