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/layouts/rightRender.tsx

@@ -82,7 +82,7 @@ export const getRightRenderContent = (opts: {
     {
       key: 'site',
       icon: <HomeOutlined />,
-      label: 'GPUStack',
+      label: 'MesaStack',
       url: externalLinks.site
     },
     {