Allow desktop app to connect to selfhosted servers (#4902)

# Description of Changes
Changes the desktop app to allow connections to self-hosted servers on
first startup. This was quite involved and hit loads of CORS issues all
through the stack, but I think it's working now. This also changes the
bundled backend to spawn on an OS-decided port rather than always
spawning on `8080`, which means that the user can have other things
running on port `8080` now and the app will still work fine. There were
quite a few places that needed to be updated to decouple the app from
explicitly using `8080` and I was originally going to split those
changes out into another PR (#4939), but I couldn't get it working
independently in the time I had, so the diff here is just going to be
complex and contian two distinct changes - sorry 🙁

frontend/src/desktop/services/apiClient.ts

@@ -0,0 +1,34 @@
+/**
+ * Desktop-specific API client using Tauri's native HTTP client
+ * This file overrides @core/services/apiClient.ts for desktop builds
+ * Bypasses CORS restrictions by using native HTTP instead of browser fetch
+ */
+
+import type { AxiosInstance } from 'axios';
+import { create } from '@app/services/tauriHttpClient';
+import { handleHttpError } from '@app/services/httpErrorHandler';
+import { setupApiInterceptors } from '@app/services/apiClientSetup';
+import { getApiBaseUrl } from '@app/services/apiClientConfig';
+
+// Create Tauri HTTP client with default config
+const apiClient = create({
+  baseURL: getApiBaseUrl(),
+  responseType: 'json',
+  withCredentials: true,
+});
+
+// Setup interceptors (desktop-specific auth and backend ready checks)
+// Cast to AxiosInstance - Tauri client has compatible API
+setupApiInterceptors(apiClient as unknown as AxiosInstance);
+
+// ---------- Install error interceptor ----------
+apiClient.interceptors.response.use(
+  (response) => response,
+  async (error) => {
+    await handleHttpError(error); // Handle error (shows toast unless suppressed)
+    return Promise.reject(error);
+  }
+);
+
+// ---------- Exports ----------
+export default apiClient;