feat: avatar decorations (P5-13)

256×256 APNG overlays from avatardecoration.com (open CORS CDN).
Stored in the user's Matrix profile as io.lotus.avatar_decoration via
MSC4133 so all Lotus Chat users see each other's decorations.

- avatarDecorations.ts: curated catalog of 110 original-IP decorations
  across 9 categories (Gaming, Cyber, Space, Fantasy, Elements,
  Japanese, Nature, Spooky, Cozy)
- useAvatarDecoration: per-user profile fetch with module-level cache
  and in-flight deduplication so concurrent renders for the same userId
  share one HTTP request
- AvatarDecoration: position:relative wrapper that overlays the APNG
  8px beyond the avatar on all sides; renders nothing when no decoration
  is set (zero cost for undecorated users)
- ProfileDecoration: scrollable grid picker in Settings → Profile,
  grouped by category with live preview; Save button appears only when
  the selection differs from what's saved
- Applied at all five avatar display sites: message timeline, members
  drawer, knock list, @mention autocomplete, notifications inbox

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

src/app/features/settings/account/Profile.tsx

@@ -48,6 +48,7 @@ import { createUploadAtom, UploadSuccess } from '../../../state/upload';
 import { CompactUploadCardRenderer } from '../../../components/upload-card';
 import { useCapabilities } from '../../../hooks/useCapabilities';
 import { useUserPresence } from '../../../hooks/useUserPresence';
+import { ProfileDecoration } from './ProfileDecoration';
 import { EmojiBoard } from '../../../components/emoji-board';
 
 type ProfileProps = {
@@ -892,6 +893,7 @@ export function Profile() {
         <ProfileStatus />
         <ProfilePronouns />
         <ProfileTimezone />
+        <ProfileDecoration />
       </SequenceCard>
     </Box>
   );