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/apiClientConfig.ts

@@ -2,16 +2,19 @@ import { isTauri } from '@tauri-apps/api/core';
 
 /**
  * Desktop override: Determine base URL depending on Tauri environment
+ *
+ * Note: In Tauri mode, the actual URL is determined dynamically by operationRouter
+ * based on connection mode and backend port. This initial baseURL is overridden
+ * by request interceptors in apiClientSetup.ts.
  */
 export function getApiBaseUrl(): string {
   if (!isTauri()) {
     return import.meta.env.VITE_API_BASE_URL || '/';
   }
 
-  if (import.meta.env.DEV) {
-    // During tauri dev we rely on Vite proxy, so use relative path to avoid CORS preflight
-    return '/';
-  }
-
-  return import.meta.env.VITE_DESKTOP_BACKEND_URL || 'http://localhost:8080';
+  // In Tauri mode, return empty string as placeholder
+  // The actual URL will be set dynamically by operationRouter based on:
+  // - Offline mode: dynamic port from tauriBackendService
+  // - Server mode: configured server URL from connectionModeService
+  return '';
 }