Add TypeScript types to utils.ts (issue #17)

- Add types to all function parameters and return values
- Replace getEl() with native document.querySelector() throughout
- Fix removeEventListener bug (was removing wrong function reference)
- Fix disabled property: use boolean true/false instead of strings
- Add ElementHandlerConfig labeled tuple type for conditionalElementHandler

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

games/static/js/add_purchase.js

@@ -1,4 +1,4 @@
-import { getEl, disableElementsWhenTrue, onSwap } from "./utils.js";
+import { disableElementsWhenTrue, onSwap } from "./utils.js";
 
 const RELATED_PURCHASE_URL = "/tracker/purchase/related-purchase-by-game";
 
@@ -11,7 +11,7 @@ document.addEventListener("search-select:change", (event) => {
   const last = event.detail.last;
   const platformId = last && last.data ? last.data.platform : "";
   if (platformId) {
-    const platformEl = getEl("#id_platform");
+    const platformEl = document.querySelector("#id_platform");
     if (platformEl) platformEl.value = platformId;
   }
 
@@ -26,7 +26,7 @@ document.addEventListener("search-select:change", (event) => {
     })
     .then((html) => {
       if (html === null) return;
-      const target = getEl("#id_related_purchase");
+      const target = document.querySelector("#id_related_purchase");
       if (target) target.outerHTML = html;
     });
 });