## What kind of change does this PR introduce?
Feature (shared UI primitive).
## What is the current behavior?
No shared stepped wizard shell in Studio. Upcoming create-pipeline work
needs a reusable step container with header, progress, and
primary/secondary actions.
## What is the new behavior?
Adds `SteppedFlow` and `SteppedFlowHeader` with unit tests: step list,
current step content, next/back, optional first-step cancel, and header
actions slot.
No product callsite yet. Safe to merge on its own; the create-pipeline
wizard PR will consume it.
## To test
Code review + vitest:
```sh
cd apps/studio && pnpm exec vitest run components/ui/SteppedFlow/SteppedFlow.test.tsx
```
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Added a reusable stepped-flow interface with progress indicators and
step-specific content.
* Supports optional headers, actions, loading and disabled states,
forms, and configurable button types.
* Added navigation controls for moving forward, going back, cancelling,
and completing the flow.
* Supports customizable final actions.
* **Tests**
* Added coverage for navigation, cancellation, headers, optional
actions, final actions, and disabled states.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->