FTUX Onboarding doesn't scroll, and I can't hit the finish button after setting up

## Bug

On the machine onboarding flow (step 4 — "Configure your default model settings"), the "Next" button is not visible and the page cannot be scrolled to reach it. This blocks completing first-time setup entirely.

**Installed from:** MSC (internal release)
**Version:** v0.4.22 (latest)
**Platform:** macOS (corp laptop)

## Screenshot

## Expected

The docked "Next" CTA at the bottom of the screen should always be visible, regardless of content height.

## Analysis

The `OnboardingFooter` portals the CTA into a `position: fixed; bottom: 5` slot rendered by `OnboardingFooterProvider`. The slot lives inside the `MachineOnboardingFlow` container which has `max-h-dvh overflow-y-auto`.

Hypothesis: on shorter displays (or default Tauri window size from MSC install), the fixed footer slot is either clipped by the `overflow-y-auto` ancestor or trapped by a CSS transform from `OnboardingSlideTransition` establishing a containing block.

## Relevant files

- `desktop/src/features/onboarding/ui/MachineOnboardingFlow.tsx`
- `desktop/src/features/onboarding/ui/DefaultConfigStep.tsx`
- `desktop/src/features/onboarding/ui/OnboardingFooter.tsx`

## Workaround

Resize the Buzz window taller, or Tab-focus to the hidden button and press Enter.

Read MachineOnboardingFlow.tsx, DefaultConfigStep.tsx, and OnboardingFooter.tsx, starting with how OnboardingFooterProvider places the CTA inside the scrolling onboarding container. Reproduce the flow in a short or default-size macOS window, then verify that the step 4 Next button remains visible and can complete onboarding without resizing or keyboard focus.

Written by the indexing model from the issue text.