Render the vertical tabs widget server side to eliminate the flash of untransformed content

Problem/Motivation

Forms with a vertical tabs group first paint as a stack of expanded <details> elements and only become the tabs widget once JavaScript attaches at DOMContentLoaded. The flash happens on every load and grows with latency, form size, and BigPipe streaming.

Steps to reproduce

Open /admin/structure/types/manage/article in Claro and hard-reload; the settings groups render as stacked details before snapping into tabs.

Proposed resolution

Render the initial vertical tabs markup on the server.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet