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/ru-RU/billing.ts

@@ -1,7 +1,7 @@
 export default {
   'billing.upsell.title': 'Billing is an Enterprise feature',
   'billing.upsell.subtitle':
-    'Track spend, generate invoices, and enforce budgets across teams. Upgrade to GPUStack Enterprise to manage billing.',
+    'Track spend, generate invoices, and enforce budgets across teams. Upgrade to MesaStack Enterprise to manage billing.',
   'billing.upsell.featuresTitle': 'What you get in Enterprise',
   'billing.upsell.feature.usage':
     'See cost breakdowns by organization, user, and model',