chore: rebrand product as 省心推 and lock UI to zh-CN

Update server titles, web/desktop window/tray/document titles, login
copy, and electron build metadata to use the Chinese product name 省心推.
Drop the runtime locale switcher and en-US import — the app now ships
zh-CN only.

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

apps/desktop-client/electron-builder.yml

@@ -1,6 +1,6 @@
 appId: com.geo.rankly.desktop
-productName: GEO Rankly Desktop
-copyright: Copyright © 2026 GEO Rankly
+productName: 省心推
+copyright: Copyright © 2026 shengxintui.com. All Rights Reserved.
 directories:
   output: release
   buildResources: build