[mod] client/simple: client plugins (#5406)

* [mod] client/simple: client plugins

Defines a new interface for client side *"plugins"* that coexist with server
side plugin system. Each plugin (e.g., `InfiniteScroll`) extends the base
`ts 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.

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`](1ad832b1dc/client/simple/src/js/toolkit.ts (L134))
array has the plugin id.

* [mod] client/simple: rebuild static

client/simple/src/js/main/autocomplete.ts

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
-import { assertElement, http, listen, settings } from "../core/toolkit.ts";
+import { http, listen, settings } from "../toolkit.ts";
+import { assertElement } from "../util/assertElement.ts";
 
 const fetchResults = async (qInput: HTMLInputElement, query: string): Promise<void> => {
   try {