Resolve plugin directory based on USE_FRONTEND_BACKUP so /api/plugins
works with both frontend-backup and built frontend, aligning with
index.ts.
Ensure /app/frontend/plugins is created during non-backup Docker
builds to avoid missing-directory issues at runtime.
Update backup asset import reference and adjust local helper
script path.
Add setupPluginRoutes to the server and register it in index.ts.
Include /furryplace-sdk.js on admin, index and moderation backup pages
to enable custom button extensions.