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/ChatContainer.tsx

@@ -40,7 +40,7 @@ export default function ChatContainer() {
         <div className="max-w-2xl mx-auto px-4 py-6">
           {messages.length === 0 ? (
             <div className="flex flex-col items-center justify-center h-full text-center space-y-2 pt-32">
-              <div className="w-16 h-16 rounded-[20px] bg-[var(--mx-primary)] flex items-center justify-center shadow-lg shadow-[var(--mx-primary)]/20 mb-2">
+              <div className="w-16 h-16 rounded-[20px] overflow-hidden shadow-lg mb-2" style={{ boxShadow: '0 0 24px color-mix(in srgb, var(--mx-primary) 20%, transparent)' }}>
                 <img src="/images/cyrene-avatar.png" alt="Cyrene" className="w-full h-full object-cover" />
               </div>
               <p className="text-[var(--mx-text-dim)] text-sm">开始新对话吧~</p>