feat(public): add setting dark or light mode

app/composables/useTheme.ts

@@ -0,0 +1,31 @@
+import { ref, watchEffect } from 'vue'
+
+const THEME_KEY = 'theme-mode'
+
+export function useTheme() {
+  const theme = ref<'light' | 'dark'>(getInitialTheme())
+
+  function getInitialTheme() {
+    if (typeof window === 'undefined') return 'light'
+    const persisted = localStorage.getItem(THEME_KEY)
+    if (persisted === 'dark' || persisted === 'light') return persisted
+    // fallback: system preference
+    return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'
+  }
+
+  function setTheme(newTheme: 'light' | 'dark') {
+    theme.value = newTheme
+    localStorage.setItem(THEME_KEY, newTheme)
+    document.documentElement.classList.toggle('dark', newTheme === 'dark')
+  }
+
+  function toggleTheme() {
+    setTheme(theme.value === 'dark' ? 'light' : 'dark')
+  }
+
+  watchEffect(() => {
+    setTheme(theme.value)
+  })
+
+  return { theme, toggleTheme }
+}