feat: 添加主题管理功能，更新系统设置页面，优化用户体验

src/composables/useTheme.ts

@@ -0,0 +1,26 @@
+import { usePreferredDark } from "@vueuse/core";
+
+export type ThemeMode = "light" | "dark" | "auto";
+
+const STORAGE_KEY = "app-theme-mode";
+
+export function useTheme() {
+  const prefersDark = usePreferredDark();
+  const theme = useStorage<ThemeMode>(STORAGE_KEY, "auto");
+
+  const isDark = computed(() => {
+    if (theme.value === "auto") {
+      return prefersDark.value;
+    }
+    return theme.value === "dark";
+  });
+
+  watch(isDark, (dark) => {
+    document.documentElement.classList.toggle("ion-palette-dark", dark);
+  }, { immediate: true });
+
+  return {
+    theme,
+    isDark,
+  };
+}