feat(dependency-graph): implement Phase 1 impact analysis

- Add useImpactAnalysis hook for traversing dependents
- Add custom GraphNode component with risk badges and state styling
- Add ImpactPanel slide-out with direct/transitive dependents
- Add GraphToolbar with show all/filter controls
- Refactor to modular component structure
- Default to top 15 files by importance
- Click-to-highlight with cascade animation
- LR layout for better import flow visualization

Author: DevanshuNEU