adjust minor test config: global mock of HTML functions and addition of test-id

src/lib/components/ExpandableForm.svelte

@@ -22,6 +22,7 @@
 <div data-testid="expand-container" class="flex flex-col items-center">
   <!-- + / × button -->
   <button
+    data-testid="expand-button"
     class="flex items-center justify-center w-12 h-12 rounded-full bg-blue-600 text-white text-2xl font-bold hover:bg-blue-700 transition"
     on:click={toggle}
     aria-expanded={expanded}