feat(phase2): add Flow Builder UI and internal flow routes

- Add FlowBuilder.tsx with React Flow visual editor
- Add FlowList.tsx for flow management
- Add /internal/flow/send endpoint for flow-engine messaging
- Add reactflow dependency to frontend

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/package.json

@@ -18,7 +18,8 @@
     "axios": "^1.7.9",
     "zustand": "^5.0.3",
     "socket.io-client": "^4.8.1",
-    "dayjs": "^1.11.13"
+    "dayjs": "^1.11.13",
+    "reactflow": "^11.11.4"
   },
   "devDependencies": {
     "@types/react": "^18.3.18",