#85 404: emit branded 404.html and serve it on unknown paths

What

dist/ has no 404.html, and serve falls back to a bare stub (src/serve.rs:418) that differs from built pages:

• no footer/brand links
• stale nav (shows tasks even when the built nav hides it)
• only not found / Nothing here.

On Cloudflare Pages and nginx, unknown paths show the host's default 404 instead of the site.

Reproduced

ls /tmp/rev/404.html          # missing
curl -s http://127.0.0.1:39320/nope   # stub page

Impact

Breaks the "one shape" requirement and looks broken at the most visible failure state.

Fix

• Emit a branded 404.html through the normal layout (nav/footer/provenance; empty-ish body).
• serve: for unknown paths, serve 404.html with status 404 if present; keep the stub only as a fallback.
• Ensure GET / and real files still 200; add tests for status + footer + nav consistency.