[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>

tests-ui/tests/renderer/thumbnail/composables/useWorkflowThumbnail.spec.ts

@@ -6,7 +6,7 @@ import {
   useWorkflowStore
 } from '@/platform/workflow/management/stores/workflowStore'
 
-vi.mock('@/renderer/thumbnail/graphThumbnailRenderer', () => ({
+vi.mock('@/renderer/core/thumbnail/graphThumbnailRenderer', () => ({
   createGraphThumbnail: vi.fn()
 }))
 
@@ -22,10 +22,10 @@ vi.mock('@/scripts/api', () => ({
 }))
 
 const { useWorkflowThumbnail } = await import(
-  '@/renderer/thumbnail/useWorkflowThumbnail'
+  '@/renderer/core/thumbnail/useWorkflowThumbnail'
 )
 const { createGraphThumbnail } = await import(
-  '@/renderer/thumbnail/graphThumbnailRenderer'
+  '@/renderer/core/thumbnail/graphThumbnailRenderer'
 )
 const { api } = await import('@/scripts/api')