CUB-125: address Grimm review — tests, type fixes, error state circuit breaker

- Add missing 'offline' to AgentStatus union type (types/index.ts)
- Add max-retry circuit breaker to useSSE; error state is now reachable
- Wire typed SSE payloads (SSEPayloadMap discriminated union) into useRealtimeSync
- Add Vitest + 20 unit tests: useSSE lifecycle, back-off, circuit breaker,
  event parsing, cleanup; useRealtimeSync event-to-invalidation mapping
- Rebase on dev to remove stale CUB-119 legacy-deletion commit and align
  CI workflow (dev already consolidated into single dev.yml)
- Tests: npm test → 20/20 pass; Build: npm run build → 0 errors

frontend/package.json

@@ -7,7 +7,9 @@
     "dev": "vite",
     "build": "tsc -b && vite build",
     "lint": "eslint .",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "dependencies": {
     "@tanstack/react-query": "^5.100.9",
@@ -20,6 +22,8 @@
   "devDependencies": {
     "@eslint/js": "^10.0.1",
     "@tailwindcss/vite": "^4.2.4",
+    "@testing-library/jest-dom": "^6.9.1",
+    "@testing-library/react": "^16.3.2",
     "@types/node": "^24.12.2",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
@@ -29,10 +33,12 @@
     "eslint-plugin-react-hooks": "^7.1.1",
     "eslint-plugin-react-refresh": "^0.5.2",
     "globals": "^17.5.0",
+    "jsdom": "^29.1.1",
     "postcss": "^8.5.14",
     "tailwindcss": "^4.2.4",
     "typescript": "~6.0.2",
     "typescript-eslint": "^8.58.2",
-    "vite": "^8.0.10"
+    "vite": "^8.0.10",
+    "vitest": "^4.1.7"
   }
 }