refactor: extract standalone browser submodels

[nathanhuh]

Summary

• extract Secrets Manager, S3, and Lambda browser state/update/view logic into feature submodels
• remove the corresponding root Model fields and app.go dispatch/view/filter branches
• preserve existing list/detail/invoke behavior and update tests for the new ownership

Progresses #107.

Validation

• make test
• make build

[amazon-q-developer]

This PR successfully refactors Secrets Manager, S3, and Lambda browser functionality into standalone submodels, following the established pattern from other features. The changes cleanly separate concerns and maintain existing behavior while improving code organization.

Key Changes:

• Extracted state management into secretsModel, s3Model, and lambdaModel structs
• Moved update/view logic to dedicated screen files
• Updated filter handling to use the featureSubmodel interface
• Removed root Model field clutter

The refactoring preserves all existing functionality and test coverage. Nice work maintaining consistency with the existing submodel architecture!

---

You can now have the agent implement changes and create commits directly on your pull request's source branch. Simply comment with /q followed by your request in natural language to ask the agent to make changes.