WIP: Refactor ApplicationController into composable concerns

[goosys]

• resolves ApplicationController refactoring #2814

Summary

Refactor Administrate::ApplicationController by splitting its responsibilities into focused concerns.

The default controller includes all concerns required for the existing HTML CRUD behavior, so this change preserves the current behavior while making customization more explicit.

With this structure, custom controllers can compose only the functionality they need. For example:

• A controller that does not render HTML can omit HtmlRenderer
• Custom controllers can include only the actions they need, such as index, or new and create.
• Kaminari pagination can be replaced with a custom pagination concern
• The default search implementation can be replaced with Ransack, Elasticsearch, or another solution

I understand the concern that concerns can hide methods that still belong to the controller. In this case, however, the goal is not only to organize methods, but to make each responsibility independently composable and replaceable. The included concerns are explicitly listed in ApplicationController, and custom controllers can choose their own composition.

Existing HTML CRUD behavior and customization points are preserved.

WIP

• Validation context support is included, but has not been merged yet.
• Unit tests for each concern still need to be added.
• Documentation still needs to be updated.
• Discussion about the granularity of method extraction and module naming.

[goosys]

@pablobm @nickcharlton

I’d like to reopen this discussion. Do you have any thoughts?