fix(desktop): degrade missing avatar image to letter circle

Avatar rendered a bare <img> with no error handling, so an avatar_url whose
storage object is unreachable (e.g. a 404 after the server move) showed a
broken image instead of the coloured letter-circle fallback. Track an onError
flag and fall back to the circle; reset it when the URL changes so a fresh
valid avatar is retried. This is client-side resilience only — it does not
restore a genuinely missing storage object.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/desktop/src/components/Avatar.tsx

@@ -1,6 +1,8 @@
 // Reusable avatar that prefers an uploaded image and falls back to a coloured
 // letter circle. Use this everywhere the app needs to render a profile.
 
+import { useEffect, useState } from 'react';
+
 import { useCachedAvatarUrl } from '../lib/avatarCache';
 
 interface Props {
@@ -27,7 +29,13 @@ export function Avatar({
   loading = 'lazy',
 }: Props) {
   const effectiveUrl = useCachedAvatarUrl(url);
-  if (effectiveUrl) {
+  // If the image URL is non-empty but unreachable (e.g. the storage object is
+  // missing / 404s), the bare <img> would render broken with no fallback.
+  // Track a load error and degrade to the letter circle instead. Reset on URL
+  // change so a fresh, valid avatar is retried.
+  const [failed, setFailed] = useState(false);
+  useEffect(() => setFailed(false), [effectiveUrl]);
+  if (effectiveUrl && !failed) {
     return (
       <img
         src={effectiveUrl}
@@ -35,6 +43,7 @@ export function Avatar({
         className={'shrink-0 rounded-full object-cover ' + className}
         draggable={false}
         loading={loading}
+        onError={() => setFailed(true)}
       />
     );
   }