[Fix] Onboarding provider form starts above the viewport when user selects OpenAI Compatible

[roomote]

Opened on behalf of Elliott de Launay. View the task or mention @roomote for follow-up asks.

What problem this solves

Fixes an issue where users choosing OpenAI Compatible during onboarding could land on a vertically centered provider form with the first required fields starting above the visible viewport.

Why this change was made

The welcome provider setup screen was using the same centered layout as the short landing state. That centering breaks down for taller provider forms, so the setup view now starts at the top of the scroll area while the landing screen stays centered.

User impact

Users selecting OpenAI Compatible during onboarding now immediately see the Base URL and API Key fields instead of needing to recover from a clipped form. The welcome screen also has regression coverage to keep the landing screen centered while the provider setup screen stays top-aligned.

Screenshots

OpenAI Compatible onboarding opens with the first configuration fields visible at scroll top.

[roomote]

2 checks still pending after a 10-minute watch. See task

• GitHub check still pending after a 10-minute watch: platform-unit-test (windows-latest)
• GitHub check still pending after a 10-minute watch: platform-unit-test (ubuntu-latest)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!