feat: add admin user management with role-based access and self-protection

- Add AdminRequired middleware for admin-role routes
- Add login status check to reject disabled/locked accounts
- Add CRUD handlers (list, create, edit, delete) for users
- Add user_list and user_form templates with role/status badges
- Add nav entry and dashboard button for user management
- Add dynamic user count on admin dashboard
- Fix userIDFromSession to handle all numeric session types
- Self-protection: cannot delete/disable self or demote last admin
- Add EN/ZH i18n keys for all user management strings

templates/admin/dashboard.html

@@ -30,7 +30,7 @@
         </div>
         <div class="bg-white rounded-xl shadow-sm border border-gray-200 p-6">
             <p class="text-sm text-gray-500 uppercase tracking-wider">{{index .Tr "dash_users"}}</p>
-            <p class="text-3xl font-bold text-gray-900 mt-1">1</p>
+            <p class="text-3xl font-bold text-gray-900 mt-1">{{.UserCount}}</p>
         </div>
     </div>
 
@@ -50,6 +50,12 @@
                 class="inline-flex items-center gap-2 bg-gray-200 text-gray-700 px-6 py-3 rounded-lg font-medium hover:bg-gray-300 transition-colors">
                 {{index .Tr "article_manage"}}
             </a>
+            {{if eq .Role "admin"}}
+            <a href="/admin/users"
+                class="inline-flex items-center gap-2 bg-gray-200 text-gray-700 px-6 py-3 rounded-lg font-medium hover:bg-gray-300 transition-colors">
+                {{index .Tr "dash_user_mgmt"}}
+            </a>
+            {{end}}
             <a href="/admin/settings/site"
                 class="inline-flex items-center gap-2 bg-gray-200 text-gray-700 px-6 py-3 rounded-lg font-medium hover:bg-gray-300 transition-colors">
                 {{index .Tr "settings_nav"}}