feat(nimbus): add rollout setup progress and issues modal

[RJAK11]

Because

• users need to know their rollout setup progress

This commit

• adds a setup completion progress bar to the rollout sidebar
• adds an issues modal that lists invalid fields with links to the relevant card

Fixes #15927

Screen.Recording.2026-07-08.at.3.01.38.PM.mov

[RJAK11]

When testing locally with the "this is a rollout" checkbox checked, you'll hit 3 additional warnings not in the video for the population percent, total enrolled clients, and reference branch fields.

A few thoughts on handling these:

• Total enrolled clients doesn't apply to the new UI, and I was told we can just default it to 1 when we add the "create new rollout" flow, until we create a dedicated rollout serializer. I think the same idea works for Reference branch.
• For Population percent, the current serializer doesn't support the new rollout phases model, so I was thinking maybe we could also hardcode a value here too since it'll get overwritten anyway once the user requests to start a phase.

Separately, we might need to bring over the AI risk field from the old UI into the new Risks card, and Public description into the Overview card or we can handle them the same way since we don’t currently have those fields. Let me know what you think!