feat(branding): rebrand footer vendor to MesaStack, trim footer/topbar

- Footer vendor label unified to "MesaStack" across all locales.
- Remove footer Help button; keep version as plain text (no click).
- Remove top-right GitHub star widget (delete unused github-star.tsx);
  keep version as plain text (no click), retain upgrade "New" badge.

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

src/locales/ja-JP/common.ts

@@ -213,7 +213,7 @@ export default {
   'common.form.password': 'パスワード',
   'common.form.username': 'ユーザー名',
   'common.login.rember': 'ログイン状態を保持',
-  'settings.company': 'MesaStack.ai',
+  'settings.company': 'MesaStack',
   'common.button.help': 'ヘルプ',
   'common.button.feedback': 'フィードバック',
   'common.button.docs': 'ドキュメント',