[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/renderer/core/thumbnail/graphThumbnailRenderer.ts

@@ -6,7 +6,7 @@ import {
   calculateNodeBounds
 } from '@/renderer/core/spatial/boundsCalculator'
 
-import { renderMinimapToCanvas } from '../extensions/minimap/minimapCanvasRenderer'
+import { renderMinimapToCanvas } from '../../extensions/minimap/minimapCanvasRenderer'
 
 /**
  * Create a thumbnail of the current canvas's active graph.

src/renderer/core/thumbnail/useWorkflowThumbnail.ts

@@ -1,7 +1,8 @@
 import { ref } from 'vue'
 
 import { ComfyWorkflow } from '@/platform/workflow/management/stores/workflowStore'
-import { createGraphThumbnail } from '@/renderer/thumbnail/graphThumbnailRenderer'
+
+import { createGraphThumbnail } from './graphThumbnailRenderer'
 
 // Store thumbnails for each workflow
 const workflowThumbnails = ref<Map<string, string>>(new Map())