Add a "Not found" template target for designed 404 pages

[DavidBabinec]

What

Adds a third template target kind — Not found (404) — so a designed page can be served for every public 404, replacing the bare { "error": "Not found" } JSON response.

How it works

Core (src/core)

• TemplateTargetSchema gains { kind: 'notFound' } (+ tolerant parseTarget branch). No DB migration — template config lives in cells_json.
• New resolveNotFoundTemplate(site) picks the winner (highest priority, document order breaks ties). A notFound template never enters resolveTemplateChain — it isn't a breadth level; route matching never "matches" a 404.

Publish (server/publish)

• renderPublishedNotFound composes the template like a regular page — wrapped by the everywhere layout chain, so the 404 page carries the site chrome.
• Full publish bakes it to 404.html in the slot — deliberately the static-hosting convention, so a raw static export keeps a working error page on Netlify / GitHub Pages. Baked first, so a literal page with slug 404 would overwrite it and stay authoritative.

Serve (server/router.ts, publicRouter.ts)

• New last dispatcher route tryServeNotFoundPage: any GET no other route claimed serves the baked artefact (one disk read, no DB — the path bot probes hit) with status 404; falls back to a live render through the Layer B LRU under the reserved /404 key (stored once per publish version, shared by every missed URL). No notFound template → today's JSON 404, unchanged.
• Namespaced prefixes (/admin/api/*, /_instatic/*, /uploads/*) keep their own 404s; the fresh-install setup redirect still wins (it runs earlier).

Editor / AI

• "Not found (404)" option in Template settings → Applies to; explorer meta shows "Not found".
• Canvas wraps the template in the everywhere layout (mirrors the publish composition); "Open live page" resolves to /404.
• setPageTemplate AI tool accepts the new kind.

Known trade-off

A direct GET of /404 serves the baked artefact with status 200 — the same convention as GitHub Pages. Documented in docs/features/templates.md.

Testing

• bun test — 5385 pass (new: notFoundResponse.test.ts covering Layer A artefact serving incl. a DB-free proof, live-render fallback + caching, and the no-template null path; matching/parse/label/dialog tests extended).
• bun run build, bun run lint — clean.

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