feat(desktop-client): add settings window and switch default API to production

- Open a dedicated settings BrowserWindow with general / login / diagnostics / about tabs
- Persist openAtLogin and keepRunningInBackground via desktop-settings.json; hide main window instead of quitting when background mode is on
- Default API base URL now points to api.shengxintui.com; LoginView and SettingsView share a single constant
- Restore wangyihao session cookies on probe and silent refresh, and accept the legacy http://mp.163.com console origin
- Drop the standalone diagnostics view in favor of the settings tab

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/desktop-client/src/renderer/routes.ts

@@ -23,11 +23,8 @@ const routes: RouteRecordRaw[] = [
     component: () => import("./views/AiPlatformsView.vue"),
   },
   { path: "/accounts", redirect: "/media-platforms" },
-  {
-    path: "/diagnostics",
-    name: "diagnostics",
-    component: () => import("./views/DiagnosticsView.vue"),
-  },
+  { path: "/settings", redirect: "/" },
+  { path: "/diagnostics", redirect: "/" },
 ];
 
 export const router = createRouter({