Introduce a plugin installation and management system.
Backend: add InstalledPlugin model, plugin_service with download/install/enable/disable/uninstall, dynamic blueprint registration and frontend manifest generation, and plugins API endpoints (list/get/install/enable/disable/uninstall). Register plugins blueprint and attempt to hot-load installed plugin blueprints at app startup.
Frontend: add PluginLoader component (Vite import.meta.glob) to render plugin widgets, extend Marketplace UI to install/manage plugins, add API client methods for plugins, and update styles for the marketplace plugin UI. This enables installing plugins from GitHub/repos/zips and managing them from the app.
