feat(admin): implement initial admin dashboard and member management

This commit introduces the foundational structure for the admin dashboard, focusing on the member management feature.
Key additions include:

- A new page at `/admin/manage/members` to display and manage members.
- An `AddModal` component with a comprehensive form for creating new members, featuring validation with Zod and input
masking.
- A reusable `PhoneInput` component with country code selection, backed by a new `useCountries` composable and a full
country dataset.
- A custom, user-friendly global error page (`error.vue`) to handle application errors gracefully.
- Updated dashboard sidebar navigation to include the new member management section.
- Added recommended VS Code extensions and settings to improve developer experience.

app/composables/useCountries.ts

@@ -0,0 +1,76 @@
+import { countries, type Country } from "~/data/countries";
+
+/**
+ * 🌍 useCountries composable
+ * 提供国家相关的搜索、过滤、分组、查找等功能
+ */
+export const useCountries = () => {
+  /**
+   * 获取全部国家
+   */
+  const getAll = (): Country[] => countries;
+
+  /**
+   * 按洲分组
+   */
+  const groupedByContinent = computed(() => {
+    const groups: Record<string, Country[]> = {};
+    for (const c of countries) {
+      const key = c.continent || "Unknown";
+      if (!groups[key]) groups[key] = [];
+      groups[key].push(c);
+    }
+    return groups;
+  });
+
+  /**
+   * 按名称搜索（支持多语言字段）
+   * @param query 搜索关键字
+   */
+  const search = (query: string) => {
+    if (!query) return countries;
+    const q = query.toLowerCase();
+    return countries.filter((c) =>
+      Object.values(c.name).some((name) => name.toLowerCase().includes(q))
+    );
+  };
+
+  /**
+   * 根据国家代码查找
+   * @param code ISO Alpha-2 或 Alpha-3 代码
+   */
+  const findByCode = (code: string) => {
+    const upper = code.toUpperCase();
+    return countries.find((c) => c.code === upper || c.iso3 === upper);
+  };
+
+  /**
+   * 获取特定语言的显示名称
+   * @param country 国家对象
+   * @param lang 语言代码（默认为 en）
+   */
+  const getDisplayName = (
+    country: Country,
+    lang: keyof Country["name"] = "en"
+  ) => {
+    return country.name?.[lang] || country.name.en;
+  };
+
+  /**
+   * 根据洲名筛选
+   * @param continent 洲名（如 'Asia'、'Europe'）
+   */
+  const filterByContinent = (continent: string) => {
+    const key = continent.trim().toLowerCase();
+    return countries.filter((c) => c.continent.toLowerCase() === key);
+  };
+
+  return {
+    getAll,
+    groupedByContinent,
+    search,
+    findByCode,
+    filterByContinent,
+    getDisplayName,
+  };
+};