perf(P6A.T5): pre-warm Supabase + avatar loading hints

Fire a no-await profiles query in AuthContext on session establish to absorb
cold-connection latency before the first user-triggered request. Add
loading='lazy' default to the central Avatar component so all off-screen
avatars (chat list, friends list, popovers, message senders) skip eager
Supabase Storage fetches; set loading='eager' on ConversationHeader (active
conv header) and CallParticipantTile inline imgs (both AudioContent and
VideoStub) which are always above-the-fold when visible.

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

apps/desktop/src/components/Avatar.tsx

@@ -11,6 +11,11 @@ interface Props {
   // brand-tinted fallback; callers can override (e.g. to colour-by-id).
   fallbackClass?: string;
   alt?: string;
+  // Browser loading hint. Use 'eager' for above-the-fold avatars (e.g. the
+  // active conversation header, call tiles). Defaults to 'lazy' so off-screen
+  // avatars (chat list rows, friends list, popovers) don't hammer Supabase
+  // Storage on initial render.
+  loading?: 'eager' | 'lazy';
 }
 
 export function Avatar({
@@ -19,6 +24,7 @@ export function Avatar({
   className = 'h-10 w-10',
   fallbackClass = 'bg-accent/20 text-accent',
   alt,
+  loading = 'lazy',
 }: Props) {
   const effectiveUrl = useCachedAvatarUrl(url);
   if (effectiveUrl) {
@@ -28,6 +34,7 @@ export function Avatar({
         alt={alt ?? displayName ?? ''}
         className={'shrink-0 rounded-full object-cover ' + className}
         draggable={false}
+        loading={loading}
       />
     );
   }