feat(topology): add network topology page; tighten config save check

- Interactive SRv6 topology canvas (@xyflow/react + d3-force auto-layout),
  custom nodes/edges, detail sheet, search and relayout controls
- Strict success-flag validation on /api/config save responses

package.json

@@ -15,8 +15,10 @@
     "@base-ui/react": "^1.7.0",
     "@fontsource-variable/inter": "^5.3.0",
     "@tailwindcss/vite": "^4",
+    "@xyflow/react": "^12.11.3",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
+    "d3-force": "^3.0.0",
     "lucide-react": "^1.33.0",
     "react": "^19.2.6",
     "react-dom": "^19.2.6",
@@ -27,6 +29,7 @@
   },
   "devDependencies": {
     "@eslint/js": "^10",
+    "@types/d3-force": "^3.0.10",
     "@types/node": "^24",
     "@types/react": "^19",
     "@types/react-dom": "^19",