Improve onboarding and provider setup guidance

[jeffpaul]

Refine onboarding flows to clearly guide users through configuring AI providers and enabling experiments. The goal is to reduce confusion for first-time users and make the initial setup path obvious and discoverable.

Acceptance criteria:

• Clear onboarding messaging when no provider is configured (relates to Graceful degradation when no AI provider is configured #182)
• Settings screens guide users toward the correct next step
• Language avoids technical jargon where possible
• First-time experience is understandable without external documentation

[juanfra]

Created an issue to tackle specifically the guide in the settings page here.