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/api/index.js

@@ -674,6 +674,15 @@ export const employeesApi = {
 export const businessUnitsApi = businessunitsApi
 
 // System Settings API
+export const pluginsApi = {
+  list() {
+    return api.get('/plugins')
+  },
+  setEnabled(name, enabled) {
+    return api.put(`/plugins/${name}`, { enabled })
+  }
+}
+
 export const settingsApi = {
   list(params = {}) {
     return api.get('/settings', { params })