chore: cleanup old ReactFlow files, delete package-lock.json

Removed:
- DependencyGraph/GraphNode.tsx (old ReactFlow node component)
- DependencyGraph/DirectoryNode.tsx (old ReactFlow directory node)
- DependencyGraph/GraphToolbar.tsx (old ReactFlow toolbar)
- frontend/package-lock.json (Bun only policy, OPE-9)

No remaining references to reactflow anywhere in the codebase.
CSS bundle dropped 4.2kb from removing reactflow styles.

Author: DevanshuNEU