Onboarding: make the "next quest" cards actionable

[talissoncosta]

Part of #7737. The bottom of the single-page flow.

After the first flag works, the flow offers "next quest" cards (kill switch / gradual rollout / experiment / remote config). Today they're illustrative previews.

Scope

• Build the next-quest-card components (prop-driven) with stories.
• Wire each card to its real next step for the flag (show_demo_button) — % rollout, multivariate, remote-config value, etc.

Done when

• Cards are standalone prop-driven components rendered by the page, with stories.
• Each card navigates to the real configuration for the flag; nothing faked.