feat: avatar decorations follow-up — Nextcloud CDN, sync script, docs

- Point DECORATION_CDN at Lotus Nextcloud WebDAV share instead of external
  avatardecoration.com; all 99 APNG files are self-hosted and served via
  direct DAV URL (no CORS issue for <img> elements)
- Add onError handler to AvatarDecoration.tsx to silently hide the overlay
  if a file is missing or the CDN is unreachable
- Rewrite scripts/syncDecorations.mjs: now sends HTTP HEAD requests to the
  live Nextcloud CDN (batches of 16 in parallel) and removes catalog entries
  for files that return non-2xx; empty categories are pruned automatically.
  Workflow: delete files from Nextcloud → run `npm run sync:decorations` →
  commit the updated avatarDecorations.ts. No local files needed.
- Add public/decorations/ to .gitignore; delete the 85 MB local APNG cache
  that was downloaded during development (files live on Nextcloud now)
- Add sync:decorations script to package.json
- Update LOTUS_FEATURES.md, LOTUS_TODO.md (P5-13 + P5-14 ✓), README.md
  with avatar decoration documentation and catalog sync workflow

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -5,3 +5,4 @@ devAssets
 
 .DS_Store
 .ideapackage-lock.json
+public/decorations/