refactor: migrate events management ui to shadcn

[overtrue]

Summary

• rebuild the event subscription creation dialog with shadcn dialog, select, input, and checkbox primitives while preserving validation and arn mapping
• refresh the events index page to use shadcn select, table, badge, and empty states for listing bucket notifications
• update the migration checklist to record completion of the events management ui conversion

Testing

• pnpm lint (fails: unable to download pnpm 9.12.3 because outbound registry access is blocked in the execution environment)

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https://chatgpt.com/codex/tasks/task_e_68fc9269a4148331a71a7adfe32c56cb