feat : add maintenance mode toggle in admin panel

- Backend: MaintenanceModeListener blocks non-admin API requests when
  var/maintenance flag file exists. MaintenanceController provides
  toggle (PUT /api/admin/maintenance) and public check endpoint
  (GET /api/maintenance/check).
- Frontend: Toggle button in admin page, maintenance.vue page for
  blocked users, middleware redirects non-admins to /maintenance.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/app/middleware/profile.global.ts

@@ -1,4 +1,4 @@
-import { useProfileSession, usePermissions } from "#imports";
+import { useProfileSession, usePermissions, useApi } from "#imports";
 
 export default defineNuxtRouteMiddleware(async (to) => {
   const { ensureSession, activeProfile } = useProfileSession();
@@ -12,9 +12,10 @@ export default defineNuxtRouteMiddleware(async (to) => {
     normalizedPath.startsWith("/profiles") ||
     fullPath.startsWith("/profiles") ||
     routeName.startsWith("profiles");
+  const isMaintenanceRoute = normalizedPath === "/maintenance";
 
   // Redirect to login if no active profile
-  if (!activeProfile.value && !isProfilesRoute) {
+  if (!activeProfile.value && !isProfilesRoute && !isMaintenanceRoute) {
     return navigateTo("/profiles");
   }
 
@@ -29,5 +30,13 @@ export default defineNuxtRouteMiddleware(async (to) => {
       }
     }
 
+    // Maintenance mode check for non-admin users
+    if (!isAdmin.value && !isMaintenanceRoute && !isProfilesRoute) {
+      const { apiCall } = useApi();
+      const res = await apiCall<{ enabled: boolean }>('/maintenance/check');
+      if (res.success && res.data?.enabled) {
+        return navigateTo("/maintenance");
+      }
+    }
   }
 });