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/pages/admin/SystemPage.tsx

@@ -41,10 +41,10 @@ export default function SystemPage() {
 
       {/* Plugins list */}
       <div>
-        <h3 className="text-sm font-medium text-white/60 mb-3">已加载插件 ({plugins.length})</h3>
+        <h3 className="text-sm font-medium text-[var(--mx-text-secondary)] mb-3">已加载插件 ({plugins.length})</h3>
         <div className="flex flex-wrap gap-2">
           {plugins.map((p, i) => (
-            <span key={i} className="px-3 py-1.5 bg-[#000] border border-[#404040] rounded-full text-xs text-[#F2F2F2]">
+            <span key={i} className="px-3 py-1.5 bg-[var(--mx-surface)] border border-[var(--mx-outline)] rounded-full text-xs text-[var(--mx-text)]">
               {p.name}
             </span>
           ))}
@@ -53,13 +53,13 @@ export default function SystemPage() {
 
       {/* Sessions */}
       <div>
-        <h3 className="text-sm font-medium text-white/60 mb-3">活跃会话 ({sessions.length})</h3>
+        <h3 className="text-sm font-medium text-[var(--mx-text-secondary)] mb-3">活跃会话 ({sessions.length})</h3>
         {sessions.length === 0 ? (
-          <p className="text-xs text-white/30">暂无活跃会话</p>
+          <p className="text-xs text-[var(--mx-text-dim)]">暂无活跃会话</p>
         ) : (
           <div className="space-y-1">
             {sessions.map((s: any, i) => (
-              <div key={i} className="bg-[#000] border border-[#404040] rounded-[14px] px-4 py-2 text-xs text-white/50">
+              <div key={i} className="bg-[var(--mx-surface)] border border-[var(--mx-outline)] rounded-[14px] px-4 py-2 text-xs text-[var(--mx-text-dim)]">
                 {s.session_id || s.id || '-'}
               </div>
             ))}
@@ -72,9 +72,9 @@ export default function SystemPage() {
 
 function StatRow({ label, value }: { label:string; value:string|number }) {
   return (
-    <div className="bg-[#000] border border-[#404040] rounded-[18px] px-4 py-3">
-      <p className="text-xs text-white/40 mb-1">{label}</p>
-      <p className="text-lg font-bold text-[#F2F2F2]">{value}</p>
+    <div className="bg-[var(--mx-surface)] border border-[var(--mx-outline)] rounded-[18px] px-4 py-3">
+      <p className="text-xs text-[var(--mx-text-dim)] mb-1">{label}</p>
+      <p className="text-lg font-bold text-[var(--mx-text)]">{value}</p>
     </div>
   )
 }