Introduces a plugin router registry for registering plugin-specific API routes, static resources, and extension pages. Updates the plugin manager and context to expose the router, and implements backend and frontend support for serving and displaying plugin extension pages in the WebUI. Also adds a demo extension page and static resource to the builtin plugin.
Refactors the plugin manager by extracting configuration, loader, and type definitions into separate modules under the 'plugin' directory. Introduces a new PluginLoader class for scanning and loading plugins, and updates the main manager to use modularized logic and improved type safety. This change improves maintainability, separation of concerns, and extensibility for plugin management.
