Remove the bespoke SelectableFilter widget

FilterSelect fully replaces it: delete SelectableFilter and its _selectable_*
helpers, the now-unused _get_filter_options, selectable_filter.js, and the .sf-*
rules in input.css (rebuilt base.css). The three list views load search_select.js
instead of selectable_filter.js. Drop the SelectableFilter export and refresh
docs/comments that referenced it.

https://claude.ai/code/session_01XzhXvMvw42CQGc9kmin3GS

games/views/game.py

@@ -149,7 +149,7 @@ def list_games(request: HttpRequest, search_string: str = "") -> HttpResponse:
         content,
         title="Manage games",
         scripts=ModuleScript("range_slider.js")
-        + ModuleScript("selectable_filter.js")
+        + ModuleScript("search_select.js")
         + ModuleScript("filter_bar.js"),
     )