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>

README.md

@@ -60,6 +60,7 @@ The Lotus Chat logo (`lotus_chat.png`) is a derivative work based on the origina
 - 20+ static chat background patterns
 - 5 animated chat backgrounds: Digital Rain, Star Drift, Grid Pulse, Aurora Flow, Fireflies (with improved per-layer looping, phosphor-flicker rain, fluid aurora sweep, and organic firefly bioluminescence)
 - 11 seasonal & holiday theme overlays — Halloween, Christmas, New Year, Autumn, Valentine's Day, St. Patrick's Day, Earth Day, Lunar New Year, April Fools', Deep Space, and Retro Arcade; auto-selected by date with a manual override in Settings → Appearance
+- Avatar decorations — 99 animated APNG overlays (Gaming, Cyber, Space, Fantasy, Nature, Spooky, Cozy, and more) that frame your avatar across the timeline, members list, and @mention autocomplete; visible to all Lotus Chat users; select in Settings → Account → Avatar Decoration
 - Toggle to pause background animations
 - Glassmorphism sidebar — frosted glass effect that lets the background show through
 - Night Light / blue light filter with an adjustable intensity slider