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.node.json

@@ -20,5 +20,5 @@
     "erasableSyntaxOnly": true,
     "noFallthroughCasesInSwitch": true
   },
-  "include": ["vite.config.ts"]
+  "include": ["vite.config.ts", "vitest.config.ts"]
 }