[ISSUE #690] Stop rendering mock consumer groups

[Aias00]

Summary

• remove production-looking local consumer group, subscription, instance, and progress mock data from GroupManagement
• show an explicit unavailable empty state until a real consumer group management API is connected
• update tests so mock group names are no longer treated as expected behavior

Fixes #690

Tests

• npm test -- GroupManagement.test.tsx
• npx eslint src/pages/studio/GroupManagement.tsx src/pages/studio/tests/GroupManagement.test.tsx src/i18n/translations.ts

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