feat: Add IAM ListUsers support

[YoungJinJung]

Summary

Add support for listing IAM users in the TUI.

Details

• Add IAM service and ListUsers feature to the catalog (internal/domain/)
• Implement ListUsers in internal/services/aws/iam.go using aws-sdk-go-v2/service/iam
• Add screen transition for the IAM feature in internal/app/actions.go
• Display user details (username, ARN, creation date, last activity, etc.)

Checklist

• Add AwsService / FeatureKind constants in internal/domain/model.go
• Register in catalog via internal/domain/catalog.go
• Implement AwsRepository.ListUsers() in internal/services/aws/iam.go
• Add screen transition in internal/app/actions.go
• Add new screen model if needed in internal/app/screens.go
• Add iam SDK dependency in go.mod
• Write tests