feat(ui): add light theme with system auto-switch + Cyrene avatar

- Miuix day/night dual theme CSS variables
- themeStore: light default, follows prefers-color-scheme
- Header: sun/moon toggle button
- All components migrated to CSS variables
- Cyrene avatar image replaces 'C' logo everywhere
- Favicon added

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

@@ -40,10 +40,10 @@ 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-[#277AF7] flex items-center justify-center shadow-lg shadow-[#277AF7]/20 mb-2">
-                <span className="text-2xl font-bold text-white">C</span>
+              <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">
+                <img src="/images/cyrene-avatar.png" alt="Cyrene" className="w-full h-full object-cover" />
               </div>
-              <p className="text-white/50 text-sm">开始新对话吧~</p>
+              <p className="text-[var(--mx-text-dim)] text-sm">开始新对话吧~</p>
             </div>
           ) : (
             <MessageList messages={messages} />
@@ -53,7 +53,7 @@ export default function ChatContainer() {
       </div>
 
       {/* Input bar */}
-      <div className="shrink-0 border-t border-[#393939] bg-[#000]">
+      <div className="shrink-0 border-t border-[var(--mx-divider)] bg-[var(--mx-surface)]">
         <div className="max-w-2xl mx-auto px-4 py-3">
           <ChatInput value={inputText} onChange={setInput} onSend={handleSend} disabled={isStreaming} />
         </div>