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/stores/serverStore.ts

@@ -1,5 +1,6 @@
 import { create } from 'zustand';
 import { persist } from 'zustand/middleware';
+import { queryClient } from '@/lib/queryClient';
 
 interface ServerStore {
   serverUrl: string;
@@ -31,15 +32,11 @@ interface ServerStore {
 }
 
 /**
- * Invalidate all React Query caches and reset UI selection state.
- * Called when the server URL changes so stale data from the previous
- * server is not shown.
+ * Invalidate all React Query caches so stale data from the previous
+ * server is not shown. Called when the server URL changes.
  */
 function invalidateAllServerData() {
-  // Lazy import to avoid circular dependency (main.tsx -> serverStore -> main.tsx)
-  import('@/main').then(({ queryClient }) => {
-    queryClient.invalidateQueries();
-  });
+  queryClient.invalidateQueries();
 }
 
 export const useServerStore = create<ServerStore>()(