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/layout/AppLayout.tsx

@@ -1,41 +1,51 @@
 import { ReactNode } from 'react'
 import { Link, useLocation } from 'react-router-dom'
 import { useAuthStore } from '../../store/authStore'
+import { useThemeStore } from '../../store/themeStore'
 
 export default function AppLayout({ children }: { children: ReactNode }) {
   const { username, logout } = useAuthStore()
+  const { theme, toggle } = useThemeStore()
   const location = useLocation()
 
   return (
-    <div className="h-screen flex flex-col bg-[#242424] text-[#F2F2F2]">
+    <div className="h-screen flex flex-col text-[var(--mx-text)]" style={{ background: 'var(--mx-bg)' }}>
       {/* Header */}
-      <header className="h-14 shrink-0 flex items-center justify-between px-4 border-b border-[#393939] bg-[#000]">
+      <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 bg-[#277AF7] flex items-center justify-center">
-            <span className="text-sm font-bold text-white">C</span>
+          <div className="w-7 h-7 rounded-lg overflow-hidden flex items-center justify-center" style={{ background: 'var(--mx-primary)' }}>
+            <img src="/images/cyrene-avatar.png" alt="Cyrene" className="w-full h-full object-cover" />
           </div>
           <span className="font-semibold text-base">Cyrene</span>
         </Link>
 
         <nav className="flex items-center gap-1">
           <Link to="/chat"
-            className={`px-3 py-1.5 rounded-full text-sm transition-colors ${
-              location.pathname.startsWith('/chat')
-                ? 'bg-[#277AF7]/15 text-[#4788FF]'
-                : 'text-white/50 hover:text-white/80'
-            }`}
+            className="px-3 py-1.5 rounded-full text-sm transition-colors"
+            style={{
+              background: location.pathname.startsWith('/chat') ? 'color-mix(in srgb, var(--mx-primary) 15%, transparent)' : 'transparent',
+              color: location.pathname.startsWith('/chat') ? 'var(--mx-tertiary-text)' : 'var(--mx-text-dim)',
+            }}
           >聊天</Link>
           <Link to="/admin/dashboard"
-            className={`px-3 py-1.5 rounded-full text-sm transition-colors ${
-              location.pathname.startsWith('/admin')
-                ? 'bg-[#277AF7]/15 text-[#4788FF]'
-                : 'text-white/50 hover:text-white/80'
-            }`}
+            className="px-3 py-1.5 rounded-full text-sm transition-colors"
+            style={{
+              background: location.pathname.startsWith('/admin') ? 'color-mix(in srgb, var(--mx-primary) 15%, transparent)' : 'transparent',
+              color: location.pathname.startsWith('/admin') ? 'var(--mx-tertiary-text)' : 'var(--mx-text-dim)',
+            }}
           >管理</Link>
-          <div className="w-px h-4 bg-[#393939] mx-1" />
-          <span className="text-xs text-white/40">{username}</span>
+          <div className="w-px h-4 mx-1" style={{ background: 'var(--mx-divider)' }} />
+          {/* Theme toggle */}
+          <button onClick={toggle}
+            className="w-8 h-8 rounded-full flex items-center justify-center text-lg transition-colors hover:opacity-80"
+            title={theme === 'dark' ? '切换日间模式' : '切换夜间模式'}
+          >
+            {theme === 'dark' ? '☀️' : '🌙'}
+          </button>
+          <span className="text-xs" style={{ color: 'var(--mx-text-dim)' }}>{username}</span>
           <button onClick={logout}
-            className="px-3 py-1.5 rounded-full text-sm text-white/40 hover:text-[#F12522] transition-colors"
+            className="px-3 py-1.5 rounded-full text-sm transition-colors"
+            style={{ color: 'var(--mx-text-dim)' }}
           >退出</button>
         </nav>
       </header>