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/tsconfig.app.json

@@ -4,7 +4,7 @@
     "target": "es2023",
     "lib": ["ES2023", "DOM"],
     "module": "esnext",
-    "types": ["vite/client"],
+    "types": ["vite/client", "vitest/globals"],
     "skipLibCheck": true,
 
     /* Bundler mode */