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/apiClientSetup.ts

@@ -0,0 +1,44 @@
+import { AxiosInstance } from 'axios';
+import { alert } from '@app/components/toast';
+import { setupApiInterceptors as coreSetup } from '@core/services/apiClientSetup';
+import { tauriBackendService } from '@app/services/tauriBackendService';
+import { createBackendNotReadyError } from '@app/constants/backendErrors';
+import i18n from '@app/i18n';
+
+const BACKEND_TOAST_COOLDOWN_MS = 4000;
+let lastBackendToast = 0;
+
+/**
+ * Desktop-specific API interceptors
+ * - Reuses the core interceptors
+ * - Blocks API calls while the bundled backend is still starting and shows
+ *   a friendly toast for user-initiated requests (non-GET)
+ */
+export function setupApiInterceptors(client: AxiosInstance): void {
+  coreSetup(client);
+
+  client.interceptors.request.use(
+    (config) => {
+      const skipCheck = config?.skipBackendReadyCheck === true;
+      if (skipCheck || tauriBackendService.isBackendHealthy()) {
+        return config;
+      }
+
+      const method = (config.method || 'get').toLowerCase();
+      if (method !== 'get') {
+        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.wait', 'Please wait for the backend to finish launching and try again.'),
+            isPersistentPopup: false,
+          });
+        }
+      }
+
+      return Promise.reject(createBackendNotReadyError());
+    }
+  );
+}