Polish super sidebar collapse animation

What does this MR do and why?

Polishes the super sidebar collapse-to-rail animation and fixes a couple of icon-only rendering issues:

• Keeps the smooth width transition when collapsing the sidebar to the icon-only rail, but clips the sidebar wrapper so the active nav item's accent background no longer breaks out past the rail edge during the animation. The left side of the clip is left open so the accent isn't shaved off.
• Centers the group/project overview avatar when the sidebar is in icon-only mode, by only applying the negative right margin in the expanded state.

Expanding continues to use the existing smooth transition and is unaffected.

References

Screenshots or screen recordings

Screen recordings of the collapse animation to be added.

Before	After

How to set up and validate locally

1. Visit any project or group page with the super sidebar (e.g. a project overview).
2. Select an item in the sidebar so it is the active/selected nav item.
3. Collapse the sidebar to the icon-only rail using the toggle.
4. Confirm the active item's accent background stays contained within the rail throughout the animation (no "break out" over the page content) and does not flicker.
5. On the group/project overview item, confirm the avatar is centered in the collapsed rail.

MR acceptance checklist

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