♿️(onboarding) improve OnboardingModal keyboard, screen reader, and preview content a11y

[Ovgodd]

Purpose

Improve OnboardingModal accessibility for keyboard and screen reader users:

• make the left step list behave like vertical tabs (correct roles, labels, and Tab order)
• support arrow navigation between steps without duplicate step announcements
• keep Next / Previous feedback when focus stays on those buttons
• allow optional contentAlt when step preview media is not decorative

What stays on Cunningham: initial focus on the dialog title at open, and other Modal chrome (dialog name, close button, etc.), tracked in Cunningham PR #390.

Proposal

• Use manual activation for vertical tabs: arrows move focus only; Enter, Space, and click change the active step.
• Keep the scrollable step list wrapper out of the tab order (tabIndex={-1}); only step buttons use roving tabindex.
• Link the preview panel to the active step with stable ids and labels; allow panel focus when content is not focusable.
• Add optional contentAlt on each step and expose it to assistive tech when preview content carries meaning.
• Announce step changes from Next / Previous via a polite status region (focus does not move to tabs).
• Remove onboarding-specific auto-focus on first step at open (to align with Cunningham modal behaviour).
• Remove the old panel live region and replace smooth scroll-on-step with scroll-into-view for the focused tab when needed.
• Verify keyboard/SR flows: Tab order, Arrows/Home/End, activation, Next/Previous announcements, and contentAlt linkage.