[Improve] Clarify inference provider choices

[roomote-roomote]

Opened on behalf of Bruno Bergher. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Updated the inference setup guidance with an explicit popular-provider hint.
• Start the provider picker unselected with a neutral "Pick your provider" prompt.
• Keep Continue disabled until a provider is selected, while preserving provider-specific setup flows.
• Restore OpenRouter after a successful OAuth callback so the saved connection remains ready to continue.

Why this change was made

The setup screen previously led with OpenRouter as the active selection and did not make the available provider choices clear. The revised guidance helps people recognize common options before choosing a connection method.

Impact

New setups now require an intentional provider selection and clearly surface ChatGPT subscriptions and OpenRouter as popular choices. OAuth-connected OpenRouter users return to a ready-to-continue selection.

[roomote-roomote]

No new code issues found. See task

• apps/web/src/app/(onboarding)/setup/StepInferenceProvider.tsx:135: Restore the OpenRouter selection after a successful OAuth callback.

_{Reviewed 74406da}

[roomote-roomote]

Fixed the OpenRouter OAuth callback selection regression in 772f143. Successful callbacks now select OpenRouter while ordinary first visits remain neutral. The focused component test and web type check pass. Screenshot capture remains blocked because this preview has no normal path to the onboarding callback state.