feat(branding): rebrand user-facing GPUStack to MesaStack

Applied via scripts/rebrand across UI labels, page title, and all
locale strings (en/zh/ja/ru/tr). Functional references untouched:
npm package, URLs, paths, JS identifiers, GPUSTACK_ constants, and the
X-GPUStack-Model header. The backend-contract startsWith string in
llmodels/hooks keeps "GPUStack server" to stay matchable. Logo images
intentionally left unchanged for now.

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

src/locales/zh-CN/organizations.ts

@@ -1,7 +1,7 @@
 export default {
   'organizations.upsell.title': '组织是企业版功能',
   'organizations.upsell.subtitle':
-    '多租户可在团队间隔离用户、资源与配额。升级到 GPUStack 企业版即可管理组织。',
+    '多租户可在团队间隔离用户、资源与配额。升级到 MesaStack 企业版即可管理组织。',
   'organizations.upsell.featuresTitle': '企业版包含的能力',
   'organizations.upsell.feature.orgs': '创建组织来分组用户并隔离工作负载',
   'organizations.upsell.feature.members': '为每个组织管理成员与角色',