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/CallParticipantTile.tsx

@@ -291,6 +291,7 @@ function AudioContent({
             src={avatarUrl}
             alt=""
             className="relative h-full w-full rounded-full object-cover"
+            loading="eager"
           />
         ) : (
           <span
@@ -366,7 +367,7 @@ function VideoStub({
         }
       >
         {avatarUrl ? (
-          <img src={avatarUrl} alt="" className="h-full w-full rounded-full object-cover" />
+          <img src={avatarUrl} alt="" className="h-full w-full rounded-full object-cover" loading="eager" />
         ) : (
           letter
         )}