feat(ui): add theme manager and dark mode

open-isle-cli/src/utils/theme.js

@@ -0,0 +1,52 @@
+import { reactive } from 'vue'
+
+export const ThemeMode = {
+  SYSTEM: 'system',
+  LIGHT: 'light',
+  DARK: 'dark'
+}
+
+const THEME_KEY = 'theme-mode'
+
+export const themeState = reactive({
+  mode: ThemeMode.SYSTEM
+})
+
+function apply(mode) {
+  const root = document.documentElement
+  if (mode === ThemeMode.SYSTEM) {
+    root.dataset.theme = window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'
+  } else {
+    root.dataset.theme = mode
+  }
+}
+
+export function initTheme() {
+  const saved = localStorage.getItem(THEME_KEY)
+  if (saved && Object.values(ThemeMode).includes(saved)) {
+    themeState.mode = saved
+  }
+  apply(themeState.mode)
+}
+
+export function setTheme(mode) {
+  if (!Object.values(ThemeMode).includes(mode)) return
+  themeState.mode = mode
+  localStorage.setItem(THEME_KEY, mode)
+  apply(mode)
+}
+
+export function cycleTheme() {
+  const modes = [ThemeMode.SYSTEM, ThemeMode.LIGHT, ThemeMode.DARK]
+  const index = modes.indexOf(themeState.mode)
+  const next = modes[(index + 1) % modes.length]
+  setTheme(next)
+}
+
+if (window.matchMedia) {
+  window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', () => {
+    if (themeState.mode === ThemeMode.SYSTEM) {
+      apply(ThemeMode.SYSTEM)
+    }
+  })
+}