fix: delete unused hook, add hover highlighting in clustered mode

- Remove unused useDirectoryClustering.ts (dead code)
- Add hover highlighting for files in clustered mode
- Keeps helpers in index.tsx (single source of truth)

Author: DevanshuNEU

frontend/src/components/DependencyGraph/hooks/useDirectoryClustering.ts

@@ -252,6 +252,9 @@ function DependencyGraphInner({ repoId, apiUrl, apiKey }: DependencyGraphProps)
           else if (selectedImpact?.directDependents.includes(node.id)) state = 'direct'
           else if (selectedImpact?.transitiveDependents.includes(node.id)) state = 'transitive'
           else if (selectedNodeId && !selectedNodeId.startsWith('dir:')) state = 'dimmed'
+          
+          // Hover highlighting in clustered mode
+          if (hoveredFileId === node.id && state === 'dimmed') state = 'direct'
 
           flowNodes.push({
             id: node.id,