Scaffold the SvelteKit app

Single-package repo: SvelteKit at the root with adapter-node. No workspace - the pet engine is now an external project and nothing else needs one.

• Fresh SvelteKit scaffold, TypeScript, adapter-node, demo routes stripped
• Source organised by feature under src/lib (components/guestbook, server/content, server/widgets), never flat type buckets
• pnpm with packageManager pinned; builds and runs clean