Redesign: campaign landing v1 (standalone ad funnel)

Deferred from slice-9 plan review (Yan 2026-06-12). A standalone marketing funnel page for paid/UTM ad traffic, distinct from the Explore homepage: hero line + omni-input front-and-center + suggestion pills, no rail nav, CTA = "Sign up free" (waitlist retired). Thin composition of the 9.2 Explore-scene components. Route name TBD (/start was proposed). Build when there's an actual campaign to run; the UTM-variant/copy-testing campaign system is a separate later effort. Not blocking the redesign — / (Explore) already serves anon cold-start.