Initialize widget JS via onSwap helper

Port FastHTML's proc_htmx as onSwap(selector, initializeElement) in
utils.js, built on htmx.onLoad: it runs an initializer once per matching
element, on initial page load and inside every htmx-swapped fragment.

Migrate search_select.js, range_slider.js, filter_bar.js and
add_purchase.js to it, removing the hand-rolled DOMContentLoaded +
htmx:afterSwap listeners and per-element guard flags. This also fixes a
latent bug: both events passed the Event object as range_slider's
"force" parameter, so every htmx swap force-re-initialized all sliders
and stacked duplicate listeners. The collapse button's
window.initRangeSliders() call was a no-op (handles are positioned in
percentages, so hidden-init is safe) and is removed with the global.

Add e2e/test_widgets_e2e.py covering the onSwap lifecycle (initial-load
init, htmx-swap init, single-fire toggles) plus FilterSelect pills and
the add-purchase type toggle. The synthetic page in
test_search_select_e2e.py now loads htmx and search_select.js as a
module, matching the new initialization path.

https://claude.ai/code/session_01BKurBhE3Qj25p7Bfsg7EeK

common/components/filters.py

@@ -559,9 +559,11 @@ def _filter_collapse_button() -> SafeText:
         tag_name="button",
         attributes=[
             ("type", "button"),
+            # Slider handles are positioned in percentages, so initializing
+            # them while the body is hidden is safe — no re-init on reveal.
             (
                 "onclick",
-                "var b=document.getElementById('filter-bar-body');b.classList.toggle('hidden');if(!b.classList.contains('hidden')&&window.initRangeSliders)window.initRangeSliders()",
+                "document.getElementById('filter-bar-body').classList.toggle('hidden')",
             ),
             (
                 "class",