Start using ViewTransitions for MPA navigations

What does this MR do and why?

Start using ViewTransitions for MPA navigations.

Block on super_sidebar.js being loaded and exectuted, to prevent sidebar and topbar from flashing.

Related to Cross-document view transitions (#613948).

Notes:

• This only works on Chrome (and its forks) and Safari. Firefox does not yet support ViewTransitions yet, but likely to land in 2026/7, see here.
• This does not yet take care of the AI rail on the right, which is init by the main.js, which we should not make render-blocking as a whole.
• The change is gated by a user-actor feature flag view_transitions for safe testing and rollout.

Status of this MR

• 2026-08-04: experiment drafted to demo and discuss. not yet ready to merge. needs agreement.
• 2026-08-13: team agreement to give this a try. safe to test behind the user-actor feature flag.
• 2026-08-14: issue #613948 created. MR is ready for team review.

References

Cross-document view transitions (#613948)

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Use Chrome browser, or own of its forks. Or Safari, I guess 🐧 But Firefox does not (yet) support view transitions!
• Enable the view_transitions feature flag.
• While being signed in, navigate between pages. See the sidebar and topbar incl their contents stay visible for the whole time between click, nav, load and render of the new page, even on slow network / high load.
• Compare to master where you often can see the app chrome (sidebar/topbar) as empty shell, without contents.

MR acceptance checklist

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