[Studio] Add client connection detail modal

[Aias00]

Summary

• add a Detail action to the client connections table
• show selected client metadata in a modal, including cluster, type, group/topic, protocol, endpoint, language, version, and heartbeat time
• add a page regression test for opening the client detail view

Validation

• npm test -- --run src/pages/cluster/__tests__/ClientsPage.test.tsx
• npm test
• npm run lint (0 errors, 4 existing fast-refresh warnings)
• npm run build
• git diff --check

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[Aias00]

Superseded by #641. Consolidated with related runtime detail modal changes to reduce review overhead while preserving the same scoped fixes and tests.