Add web UI to enable/disable plugins

GET /api/plugins now lists all discovered plugins (enabled or not) with status;
PUT /api/plugins/<name> toggles enabled (persists to plugins.json). New
Settings > Plugins admin page with per-plugin toggles. Route changes apply on
next restart. Replaces CLI-only enable/disable for self-serve admin.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/router/routes/core.js

@@ -92,6 +92,12 @@ export default [
     component: () => import('../../views/settings/SystemSettings.vue'),
     meta: { requiresAuth: true, requiresAdmin: true }
   },
+  {
+    path: 'settings/plugins',
+    name: 'plugins',
+    component: () => import('../../views/settings/PluginsList.vue'),
+    meta: { requiresAuth: true, requiresAdmin: true }
+  },
   {
     path: 'settings/auditlogs',
     name: 'audit-logs',