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/en-US/common.ts

@@ -46,7 +46,7 @@ export default {
   'common.button.enabled': 'Enabled',
   'common.button.disabled': 'Disabled',
   'common.button.upgrade': 'Upgrade',
-  'common.enterprise.feature': 'Available in GPUStack Enterprise',
+  'common.enterprise.feature': 'Available in MesaStack Enterprise',
   'common.input.holder': 'Please enter',
   'common.validate.value': '{name} value is required',
   'common.button.edit': 'Edit',
@@ -212,7 +212,7 @@ export default {
   'common.form.password': 'Password',
   'common.form.username': 'Username',
   'common.login.rember': 'Remember me',
-  'settings.company': 'GPUStack.ai',
+  'settings.company': 'MesaStack.ai',
   'common.button.help': 'Help',
   'common.button.feedback': 'Feedback',
   'common.button.docs': 'Documentation',