fix(ui): remove blue background behind avatar image

- Remove bg-primary from avatar containers in Login/Chat/MessageList/Header
- Avatar image now fills container cleanly with object-cover
- Preserve shadow glow effect

frontend/web/src/components/chat/MessageList.tsx

@@ -38,7 +38,7 @@ function MessageBubble({ message }: { message: Message }) {
     <div className={`flex ${isUser ? 'justify-end' : 'justify-start'}`}>
       {/* Avatar for AI */}
       {!isUser && (
-        <div className="w-8 h-8 rounded-xl bg-[var(--mx-primary)] flex items-center justify-center shrink-0 mr-2 mt-1">
+        <div className="w-8 h-8 rounded-xl overflow-hidden shrink-0 mr-2 mt-1">
           <img src="/images/cyrene-avatar.png" alt="Cyrene" className="w-full h-full object-cover" />
         </div>
       )}