Editing the Header/Footer sets activePageId to '__header__'/'__footer__',
which matches no entry in `pages`. save() was serializing the live canvas
into the top-level page slots (mislabeled as page content, matching no
page) while exporting header/footer from stale stored state — auto-save
every 30s silently dropped header/footer edits.
Extract buildSavePayload() as a pure, unit-tested helper: header/footer
craft state now comes from the live canvas when that zone is being
edited (else stored state), and the top-level page fields fall back to
the landing page's stored state when a header/footer zone is active,
so page content is never clobbered or mislabeled.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
