fix: render URI buttons as anchors to fix mobile popup blocker

[christianmat]

Summary

• Fixes a customer-reported bug where the "Learn more" primary button in an Announcement opens a new tab on desktop but silently does nothing on mobile (Slack thread w/ Sarah Yang).
• Root cause: useStepHandlers.handlePrimary calls navigate(uri, target) (default window.open) after await step.complete(...). iOS Safari and Chrome Android only honor window.open(_, "_blank") while the user gesture is active — any await breaks the gesture chain and the new tab is silently blocked. Desktop browsers are lax about this, which is why it works there.
• Fix: when a step exposes a URI and the consumer hasn't overridden navigate, render the button as <a href target rel="noopener noreferrer"> instead of <button>. Native anchor clicks are never popup-blocked. onClick still fires step.complete for analytics/state. Buttons with no URI and buttons under a custom navigate prop are unchanged.

What changes

• useStepHandlers returns new primaryButtonProps / secondaryButtonProps objects that include { as: 'a', href, target, rel } when appropriate. Call sites (Announcement, Banner, Card.FlowCard, Tour.TourStep, Checklist.Collapsible, Checklist.Carousel, Checklist.Floating) spread these in place of onClick={handlePrimary}.
• FrigadeContext gains hasCustomNavigate: boolean so the hook can detect when a custom navigate was passed and fall back to button rendering.
• Button.styles.ts adds cursor: pointer and text-decoration: none to the base styles so an <a> matches a <button> in every browser. Computed font/size/weight/line-height are already inherited from the inner Text.Body2 so were already identical.
• Form's submit button is unchanged — its primary action is submission, not navigation.

Test plan

• tsc --noEmit clean
• Two new Storybook stories under Components / Announcement: PrimaryButtonAsLink (renders <a> with default navigate) and PrimaryButtonLinkWithCustomNavigate (renders <button> when consumer overrides navigate). Mock the flow via __readOnly + __flowStateOverrides.
• Headless Playwright verification confirms:
  • URI + default navigate → <a href="..." target="_blank" rel="noopener noreferrer">
  • No URI → <button>
  • URI + custom navigate → <button>
• Visual screenshots of the URI and custom-navigate variants are pixel-identical except for the underlying tag name.
• Manual mobile verification (open the deployed announcement flow on a real iOS / Android device after release).

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