feat: Add Secrets Manager browser (M3.9)

[jjjjjjeonda86]

Summary

Add AWS Secrets Manager browsing feature to the TUI, allowing users to list secrets and drill into secret details including key/value pairs and encryption key.

Details

• List secrets in the selected account/region
• Drill into secret detail view: name, key/value pairs, encryption key (KMS key ID)

Checklist

• internal/services/aws/secretsmanager.go — ListSecrets(), GetSecretValue() methods
• internal/services/aws/secretsmanager_model.go — Secret model with DisplayTitle(), FilterText()
• internal/services/aws/repository.go — add SecretsManagerClientAPI interface and client field
• internal/domain/catalog.go — register FeatureSecretsManager
• internal/app/app.go — add screenSecretList, screenSecretDetail screens with Update/View logic
• internal/services/aws/secretsmanager_test.go — mock-based tests