Replace custom dependency graph with React Flow

- Install reactflow and dagre for professional graph visualization
- Use dagre for automatic tree layout (top-to-bottom)
- Custom styled nodes with package name, version, and size
- Built-in zoom/pan controls and minimap
- Click nodes to navigate to package page
- Cleaner, more professional appearance

frontend/package.json

@@ -12,9 +12,12 @@
     "test:coverage": "vitest run --coverage"
   },
   "dependencies": {
+    "@types/dagre": "^0.7.53",
+    "dagre": "^0.8.5",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",
-    "react-router-dom": "6.28.0"
+    "react-router-dom": "6.28.0",
+    "reactflow": "^11.11.4"
   },
   "devDependencies": {
     "@testing-library/jest-dom": "^6.4.2",