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/views/settings/SettingsIndex.vue

@@ -75,6 +75,12 @@
         <p>Configure integrations and system options</p>
       </router-link>
 
+      <router-link to="/settings/plugins" class="settings-card">
+        <div class="card-icon"><Puzzle :size="28" /></div>
+        <h3>Plugins</h3>
+        <p>Enable or disable installed plugins</p>
+      </router-link>
+
       <router-link to="/settings/auditlogs" class="settings-card">
         <div class="card-icon"><FileText :size="28" /></div>
         <h3>Audit Logs</h3>
@@ -91,7 +97,7 @@
 </template>
 
 <script setup>
-import { Factory, MapPin, Tag, Package, Droplets, Monitor, Laptop, Cog, Building, Globe, Link, Settings, FileText, Users } from 'lucide-vue-next'
+import { Factory, MapPin, Tag, Package, Droplets, Monitor, Laptop, Cog, Building, Globe, Link, Settings, FileText, Users, Puzzle } from 'lucide-vue-next'
 </script>
 
 <style scoped>