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/add_purchase.js

@@ -1,4 +1,4 @@
-import { getEl, disableElementsWhenTrue } from "./utils.js";
+import { getEl, disableElementsWhenTrue, onSwap } from "./utils.js";
 
 const RELATED_PURCHASE_URL = "/tracker/purchase/related-purchase-by-game";
 
@@ -38,8 +38,9 @@ function setupElementHandlers() {
   ]);
 }
 
-document.addEventListener("DOMContentLoaded", setupElementHandlers);
-document.addEventListener("htmx:afterSwap", setupElementHandlers);
-getEl("#id_type").addEventListener("change", () => {
+onSwap("#id_type", (typeSelect) => {
   setupElementHandlers();
+  typeSelect.addEventListener("change", () => {
+    setupElementHandlers();
+  });
 });