The Discuss sidebar no longer shows an unexpected grey background when the module opens. This creates a cleaner, less distracting interface while keeping highlighting only for the active sidebar item.

== ISSUE ==

When opening the Discuss module, there is a weird grey background on the sidebar.

== After this commit ==

This commit removes that style on the element, to keep only a background on the active one.

task-3326309
part of task-3326263

---
I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

The master branch release label has been updated to 16.4 alpha. This prepares the product codebase for the next development cycle and helps teams align testing, packaging, and release planning around the new version.

This update adds a test to ensure eSignature fields that depend on restricted access settings behave correctly. It helps reduce the risk of future changes accidentally breaking secure field handling in signing workflows.

The Knowledge app now shows the external link button for many-to-one property fields with a transparent style. This makes the interface look cleaner and helps the button blend better with the surrounding field design.

Purpose
=======
Make the many2one "external link" button transparent.

Task-3208449