[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`](https://github.com/inetol/searxng/blob/1ad832b1dc33f3f388da361ff2459b05dc86a164/client/simple/src/js/toolkit.ts#L134)
array has the plugin id.

* [mod] client/simple: rebuild static

searx/plugins/infinite_scroll.py

@@ -0,0 +1,28 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# pylint: disable=missing-module-docstring
+
+import typing as t
+
+from flask_babel import gettext  # pyright: ignore[reportUnknownVariableType]
+
+from searx.plugins import Plugin, PluginInfo
+
+if t.TYPE_CHECKING:
+    from searx.plugins import PluginCfg
+
+
+@t.final
+class SXNGPlugin(Plugin):
+    """Automatically loads the next page when scrolling to bottom of the current page."""
+
+    id = "infiniteScroll"
+
+    def __init__(self, plg_cfg: "PluginCfg") -> None:
+        super().__init__(plg_cfg)
+
+        self.info = PluginInfo(
+            id=self.id,
+            name=gettext("Infinite scroll"),
+            description=gettext("Automatically loads the next page when scrolling to bottom of the current page"),
+            preference_section="ui",
+        )