Delay display of pinning buttons in sidebar

What does this MR do and why?

Delay display of pinning buttons in sidebar

This should help prevent accidental clicks when trying to navigate and resolve #416555 (closed).

Screenshots or screen recordings

Before	After
recording_1692711491	recording_1692711280

How to set up and validate locally

• Enable the "New navigation" (in your user dropdown).
• On any project or group, hover over the nav items in the left sidebar.
• The 📌 icon button should only show after a short delay.
• As long as the button isn't fully faded in, pointer events (clicks) pass through it. So a click in its area before the button is shown will navigate to that page, instead of pinning it.

MR acceptance checklist

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