feat(branding): rebrand footer vendor to MesaStack, trim footer/topbar

- Footer vendor label unified to "MesaStack" across all locales.
- Remove footer Help button; keep version as plain text (no click).
- Remove top-right GitHub star widget (delete unused github-star.tsx);
  keep version as plain text (no click), retain upgrade "New" badge.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/locales/zh-CN/common.ts

@@ -204,7 +204,7 @@ export default {
   'common.form.password': '密码',
   'common.form.username': '用户名',
   'common.login.rember': '记住我',
-  'settings.company': '数澈软件',
+  'settings.company': 'MesaStack',
   'common.button.help': '帮助',
   'common.button.feedback': '反馈',
   'common.button.docs': '文档',