Message pinning has been reorganized so it belongs more clearly within the Discuss area of Odoo. This internal cleanup should make the messaging experience easier to maintain while also reducing unnecessary screen refreshes that could affect responsiveness.

Part of task-3265211

The file preview tool has been moved out of the messaging area so it can be reused by other parts of Odoo, such as website editing and document management. This makes the preview experience more consistent across apps without changing the core business workflow.

Community PR: https://github.com/odoo/odoo/pull/119625

The `mail` module's `attachment_viewer` is useful for more than `mail` alone. In order to be able to use it in the `web_editor` module (which doesn't depend on `mail`), we move it to the `web` module.

task-3109331