fix(security): validate avatar-decoration slugs from remote profiles

The MSC4133 io.lotus.avatar_decoration value was interpolated into the
CDN URL verbatim, letting a room member steer the path/query of a request
every viewer's browser makes. Accept only slugs present in the catalog;
anything else is treated as no decoration. Unit-tested.

Fixes #64

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/hooks/useAvatarDecoration.ts

@@ -1,6 +1,7 @@
 import { useEffect, useState } from 'react';
 import { MatrixError, Method } from 'matrix-js-sdk';
 import { useMatrixClient } from './useMatrixClient';
+import { isValidDecorationSlug } from '../features/lotus/avatarDecorations';
 
 const PROFILE_FIELD = 'io.lotus.avatar_decoration';
 
@@ -51,7 +52,10 @@ function fetchDecoration(
   // all fields (incl. custom MSC4133 ones); read the decoration out of it.
   return authedRequest(Method.Get, `/profile/${encodeURIComponent(userId)}`)
     .then((res) => {
-      const val = (res[PROFILE_FIELD] as string | undefined) ?? null;
+      const rawVal = (res[PROFILE_FIELD] as string | undefined) ?? null;
+      // The remote profile field is free-form and attacker-controlled; only
+      // accept it when it names a real catalog decoration (see decorationUrl).
+      const val = rawVal && isValidDecorationSlug(rawVal) ? rawVal : null;
       cache.set(userId, val);
       return val;
     })