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/views/LoginView.vue

@@ -1,6 +1,6 @@
 <script setup lang="ts">
 import { onBeforeUnmount, onMounted, reactive, ref } from "vue";
-import { useDesktopSession } from "../composables/useDesktopSession";
+import { DEFAULT_API_BASE_URL, useDesktopSession } from "../composables/useDesktopSession";
 
 const { apiBaseURL, error, pending, setApiBaseURL, login } = useDesktopSession();
 
@@ -172,7 +172,7 @@ onBeforeUnmount(() => {
               <input
                 v-model.trim="form.apiBaseURL"
                 type="text"
-                placeholder="http://localhost:8080"
+                :placeholder="DEFAULT_API_BASE_URL"
                 spellcheck="false"
               />
             </label>