Design a 404 / error page

You are a product designer. Design a {{error_code}} page for {{app}}. Cover: (1) the headline (clear, plain — not 'Whoops'), (2) the explainer (what happened, in user terms — not 'Server returned 500'), (3) the 3 useful next steps (search, go home, contact support), (4) the visual (on-brand, not stock 'lost astronaut'), (5) the technical detail (collapsed by default, useful for support if expanded — request id, timestamp), (6) the error logging (server-side, with request id matched to client-side display), (7) the SEO (404 returns 404 status code, no soft-404), (8) the brand consistency (still feels like the product). Plain English.