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

games/static/js/utils.js

@@ -1,3 +1,28 @@
+/**
+ * @description Runs initializeElement once for each element matching selector,
+ * on initial page load and inside every htmx-swapped fragment (a port of
+ * FastHTML's proc_htmx). htmx fires htmx:load for the initial document and for
+ * each swapped-in element, so a single registration covers both; the WeakSet
+ * guarantees once-per-element initialization, replacing the old
+ * DOMContentLoaded + htmx:afterSwap + per-element guard-flag pattern.
+ * @param {string} selector
+ * @param {function(Element): void} initializeElement
+ */
+function onSwap(selector, initializeElement) {
+  const initialized = new WeakSet();
+  htmx.onLoad((swappedElement) => {
+    const elements = Array.from(htmx.findAll(swappedElement, selector));
+    if (swappedElement.matches && swappedElement.matches(selector)) {
+      elements.unshift(swappedElement);
+    }
+    for (const element of elements) {
+      if (initialized.has(element)) continue;
+      initialized.add(element);
+      initializeElement(element);
+    }
+  });
+}
+
 /**
  * @description Formats Date to a UTC string accepted by the datetime-local input field.
  * @param {Date} date
@@ -202,6 +227,7 @@ function disableElementsWhenTrue(targetSelect, targetValue, elementList) {
 }
 
 export {
+  onSwap,
   toISOUTCString,
   syncSelectInputUntilChanged,
   getEl,