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

common/components/__init__.py

@@ -59,7 +59,6 @@ from common.components.domain import (
 from common.components.filters import (
     FilterBar,
     PurchaseFilterBar,
-    SelectableFilter,
     SessionFilterBar,
 )
 
@@ -109,6 +108,5 @@ __all__ = [
     "_resolve_name_with_icon",
     "FilterBar",
     "PurchaseFilterBar",
-    "SelectableFilter",
     "SessionFilterBar",
 ]