fix: address PR #319 review feedback — health validation, error handling, queryClient decoupling

- Rust: Replace fragile body.contains("status") with proper JSON
  deserialization validating status=="healthy", model_loaded (bool),
  and gpu_available (bool) to prevent misidentifying non-Voicebox services

- Frontend: Validate health response has Voicebox-specific fields before
  marking server as ready during fallback polling

- Frontend: Discriminate port-in-use errors (poll for external server) from
  real startup failures (missing sidecar, signing issues) — surface errors
  immediately with a startupError state and Retry button in the UI

- Frontend: Set explicit startup-error state when 2-minute polling timeout
  expires so the loading screen shows actionable feedback instead of hanging

- Architecture: Extract QueryClient to standalone side-effect-free module
  (lib/queryClient.ts) to decouple serverStore from React bootstrap entrypoint

app/src/lib/queryClient.ts

@@ -0,0 +1,19 @@
+import { QueryClient } from '@tanstack/react-query';
+
+/**
+ * Shared QueryClient instance used across the app.
+ *
+ * Extracted into its own side-effect-free module so it can be imported from
+ * both the React bootstrap (main.tsx) and non-React code (stores, utilities)
+ * without pulling in ReactDOM or other bootstrap side effects.
+ */
+export const queryClient = new QueryClient({
+  defaultOptions: {
+    queries: {
+      staleTime: 1000 * 60 * 5, // 5 minutes
+      gcTime: 1000 * 60 * 10, // 10 minutes (formerly cacheTime)
+      retry: 1,
+      refetchOnWindowFocus: false,
+    },
+  },
+});