fix: 修复前端多个 bug

1. 版本号闪烁问题
   - 将版本信息缓存到 Pinia store，避免每次路由切换都重新请求
   - 添加加载占位符，版本为空时显示骨架屏

2. 管理员登录跳转问题
   - 管理员登录后现在正确跳转到 /admin/dashboard
   - 普通用户仍跳转到 /dashboard

3. Dashboard 页面空白报错
   - 修复 API 返回 null 时访问 .length 导致的 TypeError
   - 为 computed 属性添加可选链操作符保护
   - 为数据赋值添加空数组默认值

frontend/src/router/index.ts

@@ -305,9 +305,10 @@ router.beforeEach((to, _from, next) => {
 
   // If route doesn't require auth, allow access
   if (!requiresAuth) {
-    // If already authenticated and trying to access login/register, redirect to dashboard
+    // If already authenticated and trying to access login/register, redirect to appropriate dashboard
     if (authStore.isAuthenticated && (to.path === '/login' || to.path === '/register')) {
-      next('/dashboard');
+      // Admin users go to admin dashboard, regular users go to user dashboard
+      next(authStore.isAdmin ? '/admin/dashboard' : '/dashboard');
       return;
     }
     next();