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/layout/AppLayout.tsx

@@ -13,7 +13,7 @@ export default function AppLayout({ children }: { children: ReactNode }) {
       {/* Header */}
       <header className="h-14 shrink-0 flex items-center justify-between px-4 border-b mx-glass" style={{ borderColor: 'var(--mx-divider)', background: 'var(--mx-surface)' }}>
         <Link to="/chat" className="flex items-center gap-2">
-          <div className="w-7 h-7 rounded-lg overflow-hidden flex items-center justify-center" style={{ background: 'var(--mx-primary)' }}>
+          <div className="w-7 h-7 rounded-lg overflow-hidden">
             <img src="/images/cyrene-avatar.png" alt="Cyrene" className="w-full h-full object-cover" />
           </div>
           <span className="font-semibold text-base">Cyrene</span>