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/store/themeStore.ts

@@ -0,0 +1,58 @@
+import { create } from 'zustand'
+
+type Theme = 'light' | 'dark'
+
+function getSystemTheme(): Theme {
+  if (typeof window === 'undefined') return 'light'
+  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'
+}
+
+function getStoredTheme(): Theme | null {
+  try {
+    const stored = localStorage.getItem('cyrene_theme')
+    if (stored === 'light' || stored === 'dark') return stored
+  } catch {}
+  return null
+}
+
+interface ThemeState {
+  theme: Theme
+  setTheme: (t: Theme) => void
+  toggle: () => void
+}
+
+export const useThemeStore = create<ThemeState>((set, get) => ({
+  theme: getStoredTheme() || 'light',
+
+  setTheme: (t: Theme) => {
+    document.documentElement.setAttribute('data-theme', t)
+    localStorage.setItem('cyrene_theme', t)
+    set({ theme: t })
+  },
+
+  toggle: () => {
+    const next = get().theme === 'dark' ? 'light' : 'dark'
+    get().setTheme(next)
+  },
+}))
+
+// Initialize theme on app start
+if (typeof document !== 'undefined') {
+  const stored = getStoredTheme()
+  if (stored) {
+    document.documentElement.setAttribute('data-theme', stored)
+  } else {
+    // Follow system preference
+    const sys = getSystemTheme()
+    document.documentElement.setAttribute('data-theme', sys)
+  }
+
+  // Listen for system theme changes
+  window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', (e) => {
+    if (!getStoredTheme()) {
+      const sys = e.matches ? 'dark' : 'light'
+      document.documentElement.setAttribute('data-theme', sys)
+      useThemeStore.setState({ theme: sys })
+    }
+  })
+}