Auto-create default project and API key on signup

Description: Currently, a developer who signs up must manually create a project and then create an API key for that project before they can write a single line of code. These are two unnecessary steps that add friction at the highest drop-off risk point in the onboarding flow.

Proposed change: Auto-create a default project and an associated API key immediately on signup. The developer should land in a ready-to-use environment with credentials already available — no setup wizard, no manual steps.

Why this matters: Every decision a developer has to make before seeing Tambo work is a drop-off risk. Removing these two steps gets them closer to writing code and seeing the agent render a real component.

These updates have been previously discussed with @MichaelMilstead.