This PR defines a new interface for client side "plugins" that coexist with server side plugin system. Each plugin (e.g., InfiniteScroll) extends the base Plugin. Client side plugins are independent and lazy‑loaded via router.ts when their load() conditions are met. On each navigation request, all applicable plugins are instanced.

classDiagram
direction BT
class Calculator {
   constructor() 
   MathJsInstance math
   run() Promise~string | undefined~
   post(result: string) Promise~void~
}
class InfiniteScroll {
   constructor() 
   run() Promise~void~
   post() Promise~void~
}
class MapView {
   constructor(map: HTMLElement) 
   HTMLElement map
   run() Promise~void~
   post() Promise~void~
}
class Plugin {
   constructor(id: string) 
   string id
   invoke() Promise~void~
   run() Promise~unknown~
   post(result: NonNullable~Awaited~ReturnType~this["run"]~~~) Promise~void~
}

Calculator  -->  Plugin 
InfiniteScroll  -->  Plugin 
MapView  -->  Plugin 

Since these are client side plugins, we can only invoke them once DOM is fully loaded. E.g. Calculator will not render a new answer block until fully loaded and executed.

For some plugins, we might want to handle its availability in settings.yml and toggle in UI, like we do for server side plugins. In that case, we extend py Plugin instancing only the information and then checking client side if settings.plugins array has the plugin id.

Due to those changes, Vite bundling also needed some rework to properly lazy-load.

I expected to refactor search.ts as a plugin too, but the PR is already somewhat large, so I left it halfway done. No logic changes were made.

I am not entirely convinced about having to recreate the answers div from scratch on client side.

Maybe we can add a condition in the answers.html template to add the style display:none if no answer blocks are appended server side?