Draft: Add transition state to the Duo Agent Platform

What does this MR do and why?

To act as an interstitial and bridge the current UI with the new UI, I'm experimenting with an overlay that acts like a boot screen when being introduced to the Duo Agent Platform for the first time.

References

Screenshots or screen recordings

CleanShot_2025-09-03_at_13.53.37

How to set up and validate locally

Turn on the New UI toggle from the user menu

MR acceptance checklist

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