fix: real footnote numbering and labels in the DOCX viewer

Three rendering defects, all in how docx-preview's per-page model was
surfaced: fabricated 'Page N' badges from DOM index (duplicating the
document's own rendered footer), footnote references restarting at 1 on
every page, and w:footnoteRef markers dropped entirely so note bodies
had no labels. Plus a fourth found during verification: notes opened by
w:customMarkFollows symbols (*, **) were force-numbered, shifting every
real number by one.

New docxNotes.ts tags references and note bodies in the parsed model,
then numbers/links them continuously in the DOM with bidirectional
anchors, honouring custom marks and de-duplicating notes repeated
across pages. DocxView drops decorateDocxPages, trims render options
to non-defaults, and caches parseAsync per ArrayBuffer so remounts do
not re-inflate the zip. Verified against two real documents (239-note
and 13-note) matching their XML exactly; 117 frontend tests pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QUsMXzBsEJ27DjJVPSfW5o