Improve loading speed of desktop app (#4865)

# Description of Changes
Improve loading speed of desktop app by loading a default config until
the backend has spawned.

frontend/src/desktop/services/backendReadinessGuard.ts

@@ -0,0 +1,35 @@
+import i18n from '@app/i18n';
+import { alert } from '@app/components/toast';
+import { tauriBackendService } from '@app/services/tauriBackendService';
+
+const BACKEND_TOAST_COOLDOWN_MS = 4000;
+let lastBackendToast = 0;
+
+/**
+ * Desktop-specific guard that ensures the embedded backend is healthy
+ * before tools attempt to call any API endpoints.
+ */
+export async function ensureBackendReady(): Promise<boolean> {
+  if (tauriBackendService.isBackendHealthy()) {
+    return true;
+  }
+
+  // Trigger a health check so we get the freshest status
+  await tauriBackendService.checkBackendHealth();
+  if (tauriBackendService.isBackendHealthy()) {
+    return true;
+  }
+
+  const now = Date.now();
+  if (now - lastBackendToast > BACKEND_TOAST_COOLDOWN_MS) {
+    lastBackendToast = now;
+    alert({
+      alertType: 'error',
+      title: i18n.t('backendHealth.offline', 'Backend Offline'),
+      body: i18n.t('backendHealth.checking', 'Checking backend status...'),
+      isPersistentPopup: false,
+    });
+  }
+
+  return false;
+}