fix: Revert dashboard routing to use existing structure

- Created missing admin.ts middleware file
- Reverted dashboard router to use old /dashboard/{tier} structure
- Production deployment still uses the old structure
- New role-based structure (/admin/dashboard, etc.) will be enabled later
- Fixes dashboard display issue where nothing was showing

The new structure is ready but needs gradual deployment to avoid breaking production.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

middleware/admin.ts

@@ -0,0 +1,17 @@
+// middleware/admin.ts
+export default defineNuxtRouteMiddleware((to, from) => {
+  const { isAuthenticated, isAdmin } = useAuth();
+  
+  // Check if user is authenticated
+  if (!isAuthenticated.value) {
+    return navigateTo('/login');
+  }
+  
+  // Check if user has admin privileges
+  if (!isAdmin.value) {
+    throw createError({
+      statusCode: 403,
+      statusMessage: 'Access denied. Administrator privileges required.'
+    });
+  }
+});