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>

config/config.ts

@@ -77,7 +77,7 @@ export default defineConfig({
   antd: {
     style: 'less'
   },
-  title: 'GPUStack',
+  title: 'MesaStack',
   hash: true,
   access: {},
   model: {},