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/features/lotus/avatarDecorations.test.ts

@@ -5,6 +5,7 @@ import {
   DECORATION_CATEGORIES,
   ALL_DECORATIONS,
   decorationUrl,
+  isValidDecorationSlug,
 } from './avatarDecorations';
 
 test('decorationUrl builds a CDN png url from the slug', () => {
@@ -66,3 +67,20 @@ test('slugs use the snake_case charset (lowercase, digits, underscore)', () => {
     assert.match(decoration.slug, /^[a-z0-9_]+$/, `bad slug: ${decoration.slug}`);
   });
 });
+
+test('isValidDecorationSlug: accepts a real catalog slug', () => {
+  assert.equal(isValidDecorationSlug('joystick'), true);
+  assert.equal(isValidDecorationSlug('lotus_flower'), true);
+});
+
+test('isValidDecorationSlug: rejects a path-traversal string', () => {
+  assert.equal(isValidDecorationSlug('../../anything'), false);
+});
+
+test('isValidDecorationSlug: rejects a slug carrying a query string', () => {
+  assert.equal(isValidDecorationSlug('joystick?u=probe'), false);
+});
+
+test('isValidDecorationSlug: rejects an empty string', () => {
+  assert.equal(isValidDecorationSlug(''), false);
+});