[refactor] Move thumbnail functionality to renderer/core domain (#5586)

Move thumbnail functionality from src/renderer/thumbnail/ to src/renderer/core/thumbnail/
to align with domain-driven design architecture. Thumbnail generation is core rendering
infrastructure and belongs alongside other core renderer utilities.

Changes:
- Move useWorkflowThumbnail.ts and graphThumbnailRenderer.ts to renderer/core/thumbnail/
- Update all import paths in consuming files
- Fix relative imports within moved files

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

src/platform/workflow/core/services/workflowService.ts

@@ -8,7 +8,7 @@ import {
   useWorkflowStore
 } from '@/platform/workflow/management/stores/workflowStore'
 import { ComfyWorkflowJSON } from '@/platform/workflow/validation/schemas/workflowSchema'
-import { useWorkflowThumbnail } from '@/renderer/thumbnail/useWorkflowThumbnail'
+import { useWorkflowThumbnail } from '@/renderer/core/thumbnail/useWorkflowThumbnail'
 import { app } from '@/scripts/app'
 import { blankGraph, defaultGraph } from '@/scripts/defaultGraph'
 import { downloadBlob } from '@/scripts/utils'

src/platform/workflow/management/stores/workflowStore.ts

@@ -6,7 +6,7 @@ import { t } from '@/i18n'
 import type { LGraph, Subgraph } from '@/lib/litegraph/src/litegraph'
 import { ComfyWorkflowJSON } from '@/platform/workflow/validation/schemas/workflowSchema'
 import type { NodeId } from '@/platform/workflow/validation/schemas/workflowSchema'
-import { useWorkflowThumbnail } from '@/renderer/thumbnail/useWorkflowThumbnail'
+import { useWorkflowThumbnail } from '@/renderer/core/thumbnail/useWorkflowThumbnail'
 import { api } from '@/scripts/api'
 import { app as comfyApp } from '@/scripts/app'
 import { ChangeTracker } from '@/scripts/changeTracker'