Visually align Source Editor toolbar with current UI + bug fixes

What does this MR do and why?

The new source editor toolbar introduced a few UX changes, even though there were no functionality changes or additions. This MR aligns the source editor toolbar UI again with the current UI.

In addition to that, it also fixes a few bugs:

• Toolbar not working on mobile
• Incorrect border radii due to buttons being in multiple button groups
• Wrap button staying focused after being toggled

Screenshots or screen recordings

Before with feature flag	After with feature flag	Before and after without feature flag

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:source_editor_toolbar)

2. Go to a repository and edit a .md file in the single file editor

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.