fix: GUI startup with external server + data refresh on server switch

Two fixes for issue #312:

1. GUI stuck on loading screen when backend is already running externally
   (e.g. via python/uvicorn/Docker):

   - Rust: add HTTP health check fallback when the process on the port
     doesn't have 'voicebox' in its name. If /health responds with a
     valid Voicebox response, reuse the server instead of erroring.
   - Frontend: when startServer() fails, fall back to polling the
     health endpoint every 2s instead of permanently blocking.

2. No data refresh when switching server URLs in settings:

   - serverStore.setServerUrl() now invalidates all React Query caches
     when the URL actually changes, so profiles/history/models/stories
     are re-fetched from the new server.
   - Export queryClient from main.tsx for store-level cache invalidation.

Fixes #312

app/src/stores/serverStore.ts

@@ -30,11 +30,29 @@ interface ServerStore {
   setCustomModelsDir: (dir: string | null) => void;
 }
 
+/**
+ * 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.
+ */
+function invalidateAllServerData() {
+  // Lazy import to avoid circular dependency (main.tsx -> serverStore -> main.tsx)
+  import('@/main').then(({ queryClient }) => {
+    queryClient.invalidateQueries();
+  });
+}
+
 export const useServerStore = create<ServerStore>()(
   persist(
-    (set) => ({
+    (set, get) => ({
       serverUrl: 'http://127.0.0.1:17493',
-      setServerUrl: (url) => set({ serverUrl: url }),
+      setServerUrl: (url) => {
+        const prev = get().serverUrl;
+        set({ serverUrl: url });
+        if (url !== prev) {
+          invalidateAllServerData();
+        }
+      },
 
       isConnected: false,
       setIsConnected: (connected) => set({ isConnected: connected }),