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/Sidebar.tsx

@@ -16,10 +16,10 @@ export default function Sidebar() {
   }
 
   return (
-    <aside className="w-60 bg-[#000] border-r border-[#393939] shrink-0 flex flex-col">
+    <aside className="w-60 bg-[var(--mx-surface)] border-r border-[var(--mx-divider)] shrink-0 flex flex-col">
       <div className="p-3">
         <button onClick={handleNewChat}
-          className="w-full py-2.5 bg-[#277AF7] hover:bg-[#3482FF] active:scale-[0.98] text-white font-medium text-sm rounded-full transition-all duration-200"
+          className="w-full py-2.5 bg-[var(--mx-primary)] hover:bg-[var(--mx-primary-hover)] active:scale-[0.98] text-white font-medium text-sm rounded-full transition-all duration-200"
         >
           + 新对话
         </button>
@@ -27,11 +27,11 @@ export default function Sidebar() {
 
       <div className="flex-1 overflow-auto px-2 pb-2">
         {currentSessionId ? (
-          <div className="px-3 py-2 bg-[#242424] rounded-[14px]">
-            <p className="text-xs text-white/40 truncate">{currentSessionId.slice(0, 12)}...</p>
+          <div className="px-3 py-2 bg-[var(--mx-bg)] rounded-[14px]">
+            <p className="text-xs text-[var(--mx-text-dim)] truncate">{currentSessionId.slice(0, 12)}...</p>
           </div>
         ) : (
-          <p className="px-3 py-2 text-xs text-white/30 text-center">暂无对话</p>
+          <p className="px-3 py-2 text-xs text-[var(--mx-text-dim)] text-center">暂无对话</p>
         )}
       </div>
     </aside>