Introduce in-memory static file support and inter-plugin exports. Add MemoryStaticFile/MemoryFileGenerator types and expose staticOnMem in PluginRouterRegistry; router registry now tracks memory routes and exposes getters. Add getPluginExports to plugin manager adapters to allow plugins to call each other's exported modules. WebUI backend gains routes to serve /plugin/:pluginId/mem/* (memory files) and /plugin/:pluginId/files/* (plugin filesystem static) without auth. Update builtin plugin to demonstrate staticOnMem and inter-plugin call, and add frontend UI to open extension pages in a new window. Note: API router no longer mounts static filesystem routes — those are handled by webui-backend.
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.
