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/main.tsx

@@ -3,6 +3,7 @@ import { createRoot } from 'react-dom/client'
 import { BrowserRouter } from 'react-router-dom'
 import App from './App'
 import './index.css'
+import './store/themeStore' // Initialize theme (light mode default, follows system)
 
 createRoot(document.getElementById('root')!).render(
   <StrictMode>